A variant on Option C - what if there is an automatic back up option that's always available?  So for example, if Mithril is unavailable, then the player starts getting units with normal Long Swords and helmets, rather than the nicer, shinier ones.  In the case of the Twililight Honey Pack, perhaps they just lose the bonus.

edit - This way, a player loses a valuable bonus without being forced to redesign their units.

When this happens, the player can be informed of the lowered quality, and the reason, but they're not forced to redesign anything immediately (unless they want to, of course).

This way, a more casual player knows what's happened, and can just get on with the game. Alternately a more hard core gamer knows to jump in and tweak the unit designs.

A possible tie in to this would be to have a 'grace period' after losing a resource that represents stock piles being used up.  So for example, I lose my Mithril mines, at which stage I'm notified that supplies are running out, and I have (say) 10 turns to recapture the mines before my troop production will be affected.  This adds a number of interesting choices to a game - do I continue as planned, or retake the hives I just lost?  Do I target the enemy's army, or reduce his production ability so that his Dread Knights are less fearsome?

Yeah I see now why you are wanting to make resources give bonuses instead of prerequisites.

For reasons I put in the other thread I still think that making it a requirement is desirable.

How about making certain materials give a bonus to an item? This allows an option C type of resolution.

For instance, what if a mithril sword was an iron sword with a mithril overlay (which gives the sword a bonus)? Then if you lose mithril, your unit gets just an iron sword (notifying you that this was happening when you lost the mithril resource of course)

For a few things this does not work as well, i.e. horse mounted units vs bear mounted units. On those you would need specific defined units for each mount type. But for metal, wood or crystal I could see the advanced/rare material just being an overlay that provided an additional bonus to the item.

I think going this route (making a mithril sword be an iron sword with mithril added) gives you the ability to treat almost everything like "equipment" in the 'c' option above. And I think it is the best of the three options by far.

One problem that I see with substitution is that you will then have some dread knights running around with mithril weapons and some running around with iron weapons. This would be a bad, bad thing imho. So maybe a is the better option after all.

I don't remember which economic model was going to be used for starters...

I'd favor A mixed with C. I mean, if you don't have mithril, forget about those nice mithril armour pieces and swords. (no more Dread Knights for you) You should get a warning about substituting pieces if je still wants to build them but... well, that would be like a new unit. If instead of the lack of mithril we talk about the lack of twilight bee apiary (why do I get the feeling that some bees at Stardock are going to be in the credits of the game?), then you can still build those Dread Knights without it (ok, you could build the Dread KNights without the mithril but they would go without weapon or armour...) because it's just an extra.

Out of curiosity: the Attack, Defense and HP... is there possibility of changes/evolution? Like adding other stats (no need to get hyper realistic but yes more varied) and/or skills. Not that I don't like the simplicity of the current system.

Edit: damn me. Forgot about that if you lose a resource (mithril in this case) but it's not just a bonus but you actually get to store quantities of it... well, you have your buffer there until you reclaim the resource again.

This is a slightly unrelated thought, but...

Will units have experience in some sense?  I like the thought that my battle hardened troops will be more effective than the raw recruits fresh off the factory floor, as it were.  The thought of units have battle honours or flags to go along with their experience would be kinda cool as well.

I would just like to chime in with Winter that, as nice as simple attack and defense is, we really need more stats. If nothing else we need resistance for each element. (I think of it as in GalCiv2 defense vs lasers/missiles/projectiles.) I think that if this is not present, then the diversity of the elements themselves is lessened, it makes all the units too vanilla.

Hm to be honest, I have no problems with Option A at all [Civ4 style -> Resources have hardcore strategical value that way]...but Option C looks ok too.

Adding another step into the economic model would help this senario out. For instance:

Mithril ore is used to make mithril armor that can be stored in a warehouse.

These armor pieces are then used to make troops.

If you lose the mithril you can still make the troops for some time but will have to get back that mining node for future production (or start trading for some armor).

Your notification feedback would come from a blacksmith that says "I have run out of ore".

Variant C hands down. Just add a dash to the units name or something so we know they are substandard.

I think you can merge options B and C.

When you lose a key resource you should be given proper notice of what that means. If you're building multiples of the unit in may need to pop up a screen saying that you only have enough of the resource to produce the object in one town... slowly. Would you like to substitute a different item instead? How much of the material is in storage across the kingdom/empire for the building of a last few units with it?

Personally I also find it painfully difficult in 4x games when a resource is lost and suddenly all production on a unit is lost because you don't have the resource to complete it and must start over. So whatever you do please don't go with Option A.

Option C sounds like a good idea to mitigate the micro-management of option A. Always a good thing. Option A I'm used to from most other games, and I would be fine with it. Not a fan of Option B.

As Tourresh mentioned, make sure it's obvious from unit selection which units are lacking items from the original "blueprint".

Why not go a simple route? After the resource is lost the player is given ten (or whatever) turns during which thier "stockpile" allows them to to continue building the units. After the time is up, give them the option to modify the units to make them legal or make an entirely new one.

That or the unit just takes longer/costs more. Thats super simple, and may be the best option.

Option C, but keep the "bonus equipment" as part of the unit, just greyed out on the units profile.  If/when your empire re-aquires the resource, you can move your unit back to a city, and hit "re-supply" or something to bring that unit up to standard.

Maybe expand the idea of equipment a bit - add one-time-use equipment to unit designs like healing salves, magic scrolls, or grenades or whatnot.  Once they are used they are greyed out until you re-supply in a city.  This way the player gets used to the idea that you may need to re-supply your units to get them to 100%, and would provide a natural bonus to a defending empire (easy to re-supply one-time-use powerups).

And like Tourresh said above - a simple indicator on the unit icon so the player knows which units aren't up to snuff.

But, if the player is missing a resource for the core equipment I'd rather see option A or B, because I don't want a thousand variants of Dread Knights running around.

I like C, though I'd like to see some of B mixed in. For nonessential items - like your honey that adds 10% hp, etc - C is fine, if access to the item is lost, just build the unit without that bonus (although it'd be good to inform the player that the 10 dread knights he just queued up will lack that bonus).

For items that you don't want to simply leave out, like armor and weapon, I'd prefer B; to see the unit still get built without the resource, just slower. If your unit uses an iron sword, and losing access to iron means you either have to redesign a new unit with a bronze sword (A) or choose a replacement sword when you try to build the unit (C), either way it's painful micromanagement. And not just micromanagement when you're training the unit, but it creates more micromanagement later - units with iron swords may perform noticeably better in combat than otherwise identical units with bronze swords, so you now have to keep track of which units in your army use iron and which got the bronze, perhaps relegating the bronze sword units to city garrison or acting as a rear guard over your supply lines, while the iron sword units are the front line, first to attack. This may sound fun and tactical, but multiply it over potentially hundreds of units, and it'll just be a pain - for simplicity's sake I'd much rather know that every unit of a given type has an iron sword, even if it takes longer to build them without access to iron.

A good number, I think, is 2x build time without the resource; i.e. if my soldier with an iron weapon and iron mail normally takes 15 turns to train, he should take something like 30 turns to train without access to iron. Of couse it's more complicated than that, what if he uses an iron sword but leather armor and you lose access to iron? However the details work out, though, I think it should wind up taking about twice as long at the most without a resource. This is a big difference, it makes you want to get access to iron, but not such a massive difference that it's no longer worth building the unit if you lose access to the resource. Keep in mind that, if you temporarily lose access to iron and you need units asap, you always have the option to build an older bronze design - but unless I'm desperate, personally I'll stick with building the iron design half as fast and focus on getting that resource back.

This is a great idea too, it lets you get all your units back up to standard later if you temporarily lost access to a resource, so you don't have to worry about some units of the same type having different stats (which I'd really like to avoid having to micromanage).

I like C as well, especially if you can design units via equipping them with things that break the rules.  I think making sure we have the ability to create rulebreaking units is important to unit diversity.  If we're only deciding attack/defense and hitpoints then the idea is just to make the best we can.  For example creating a unit with a "first strike" like ability that allows them to attack first whether they are defenders or attackers.

EDIT:

I just thought of a pretty neat way to do this.

Give them a couple extra equipment slots that you can call "Special training"..

Here you can equip them with special training in things like "first strike" or what have you

This is how I'd like to see the resources handled, as it was something that always struck me as missing from Civilization - my mighty nation was completely shut down in the later stages if I lost my oil fields. Now, this made them a great strategic Target, however it didn't make any sense that I wasn't able to somehow prepare for this.
Perhaps adding a Warehouse Building that enables a small percentage of a resource or produced items to be stockpiled when not being used in Unit production would be the best method? This ensures that your not able to stockpile thousands of items and keeps resources' strategic value, while also ensuring that losing your only available Iron Mine, for example, doesn't completely remove your ability to muster some kind of defense.