One of the things I loved about Master of Magic was the variety of the armies. A single tactical battle could have members of 7 or 8 different races, all with unique abilities and different appearances, plus several different summoned magical creatures. I'm glad to hear that fantastical creatures will be in Elemental; it would be shame to waste all that beautiful design concept and gorgeous graphic engine Stardock's been teasing us with on solely human armies.
Anyway, with that in mind, I had a couple thoughts about interesting ways that summonable and controllable fantastical creatures could be integrated with the desire to give players extensive control over unit design. Warning, text wall incoming. . .
Since the game is named Elemental, why not make the creation of summonable creatures closely tied to the elements that provide the framework for the magic system in the game? Player design of a summonable creature would be a multi-step process.
- Pick a creature type. All summoned creatures would elemental magical versions of natural beasts in the game, so the first step would be to pick your base creature type (spider or troll, to use the two that are the beta). Use of the creatures already in the game would mean that no more 3D models would need built. Each base creature type would come with a base mana cost and abilities (for example, webbing for spiders or health regen for trolls).
- Pick the main element used to create the creature. This would determine what form of mana would primarily be used to summon and maintain the creature. The choice of elemental type would modify the base creature stats (for example, air spiders would be fast but have lower defense, fire spiders would have lower base damage but apply damage over time, earth spiders would get more hitpoints but be slower)It would also have other implications, such as the skin appearance on the model, what sorts of magic the summoned elemental would be resistant or susceptible to, what sort of magic spells it might have access to, and perhaps what sort of terrain it would give it an advantage or disadvantage in combat.
- Pick secondary traits, also based on elements. These secondary abilities could also be divided into elemental classes. I'd propose that elementals not be able to use secondary traits from the opposite elemental class (i.e. fire spiders can't be given water-walking, also no earth trolls doing wind sprinting). Endowing elementals with secondary traits would determine any graphical accents on the model. They would also have costs in terms of magical resources and mana for summoning and maintenance.
Since Kyro indicated that fantastical creatures are meant to be uncommon things, I would propose that elementals only be summonable by the sovereign and that their maintenance cost vary depending on their distance from the sovereign. This would provide a prospective payoff for players who are using their sovereign for primarily magical, rather than combat, abilities to move their sovereign into riskier areas on offense (particularly if the model of the sovereign being at risk when outside the empire's zone of influence is adopted). Also, assuming human armies will gain experience, I don't think summoned elementals should; if you want a better elemental, you have to spend more mana, get access to better magical reagents, study that spellbook and summon a better one.
As a final side note, it would be fun to let a sovereign with proper specs/research/experience take control of a limited number of existing non-elemental magical creatures, with the same restrictions as elementals. Allowing this power would make creature dens and dungeons another sort of natural resource. The sovereign would, of course, have to travel to the creature's location to tame or take control of it.
Anyway, wall of text concluding, thanks for reading my ramble.