Cooperative Hero and building game

One solution might be to get rid of the building and archaeology, having players only fighting monsters. That way players can decide whether they want to team up or go solo but in the end everyone needs to be ready for the Boss. By removing different play styles the game would be a lot easier and still players can choose different combat types. It would, however, not be much different from many already existant games.

Absolutely no problem, Zag. You made a valid point, the different strategies need to be balanced really well or either some are more interesting to play or more powerful. And like you said the strategy of "everyone chooses one strategy" should not be more powerful as "pick what you like", because the whole idea would be that players can play what they like and every play style is of equal value to the end game.

@JRC: I don't think I would like to implement a balance check after every round. Rather, players have a limited number of rounds to prepare for a final fight.

@Kos: there are so many valuable items in your post, thanks! I will definitely create interdependencies between play styles, such as acquisition of spells only via artefacts and of items only via monsters, monsters not slain joining the big bad, and an important artefact boosting the big bad if the players have not found it.
Like you said it probably has to be kept simple or the game will be of enormous length. Perhaps the big bad should turn up after 15 rounds, if players can get to an artefact or monster every 2 out of 3 turns they can fight defeat ~10 of those, which may give them give a number of spells and items (unless all of 4 players go fight the same monsters, then they have to divide those 10 items over 4 players).

It's a very nice idea if players need things from different "professions". Like you say the builder can be one to acquire food, but he might run low on gold for building, which the archaeologists might supply (artefacts are of great worth ofc), and materials, which both archaeologists and combatants might find. Combatants can find items (tools/weapons, apparel, trinkets and potions) which might also be useful for builders and archeologists (e.g. a hammer can speed up the building process and a brush the digging process), and the same is true for spell scrolls, which might only be found through excavation (e.g. a fireball spell which can help in combat or a scroll with a motivational speech which speeds up building).
The harvesting of mithril you suggest is also cool - I guess you mean that because an adventuring player is in the mine he can easily acquire mithril, whereas it would be an arduous task for a builder just to get there.

I figured out the third element of the game, thus the basics are there.

1. Combat. Players can fight (groups of) monsters, which gain them gold and items. Item cards are drawn from a stack of random item cards and include weapons (hand items), apparel (clothing items) and trinkets (accessory items). There are items for all three game facets, but items can only be acquired through combat and shopping (see below).
2. Labyrinths. Archaeology is replaced by a Frogger-like minigame. Players have to traverse a treasure room/tomb of which the walls move unpredictably. At the end they will find a scroll, which they draw from a stack of random scroll cards. Scrolls include health and mana increases (upgrade then discard) and spells for all three game facets. Scrolls can also be acquired through shopping.
3. Alchemy. Players can harvest several herbs at the non-dungeon tiles in the game. The player rolls 3D10 for the terrain type he is in, which gives him specific herbs. Every terrain type (e.g. forest or mountain) has a specific subset of herbs that can be found there. There is a potion list with the requirements and effects of all potions. Players can create potions (pick the specific card) when they pay the right herbs. There will be potions for all three facets of the game.

Villages in the game sell items and scrolls (perhaps also potions). When a player enters a village he draws both an item and a scroll card, which he can buy for a fixed amount. The ones he does not buy are either put back into the deck, which is then shuffled, or discarded (I have not decided yet). Since there are no levels for the items and scrolls and drop-chance is independent of monster-difficulty, all items and spells will all cost the same.

When players enter a dungeon they draw a card, which depicts what they encounter. This can be either monsters or labyrinths, and the card also shows what kind of monsters or labyrinth they face.
Once they complete this, they can go to the second out of three levels of the dungeon, which will be more difficult. Because players gain more items as they progress, they can face the second and third level easier once the game progresses (or if they're in a group).
Since items and spells are drawn randomly from single stacks, I do not know how to increase the reward on second and third levels. Suggestions?

Please let me know what you think.