Wall of text... eventually about demons:

I was just playing x-com apocalypse, one of my favourite old games. Still great: tactical combat, turn based or real time, variable sized overland map, an economy of sorts, diplomacy, rival factions, multiple maps, rpg-like exp gain, research, manufacture and base building...etc

One thing that struck me, I really like researching toxic weapons, how you have to learn about each alien first before you're able to develop a weapon against them. It's not required, but once you capture an alien queen you get a mega weapon of alien gas grenades. Something for when you're just about to win anyway, perhaps there could have been alien gas grades A, B and C to get more fun out of it... but that doesn't matter here... back to elemental. Ok, so no aliens in elemental (or grenades? holy handgrenades maybe?)... but there are demons, monsters and odd creatures. How about if each defeated monster is able to provide a corpse for magical study? capturing a live demon allows you to study it... you'd have to get it weak enough to capture first... then bind it with spells. It would allow you to grow more powerful incrementally in demonic arts... capture a giant spider, wandering troll, demon bat... then unlock some demon summoning spells for the evil sov or demon banishing spells for those that way inclined... Perhaps it could be a choice, like the evil/neutral/good choices in GalCiv. Attacking a monster with an army, weakening, binding it and then drawing in a tactical battle (obviously not killing it or losing outright) could trigger an event, the results of which unlock a research option and they in turn unlock spell options.

What I would like to make it a little different, rather than simply researching for 10 turns and being able to summon demons, to advance up the magical path would take lots of little demon encounters. In x-com this was done by having to sample one of each alien, aliens with linked life-cycles opened up research after all were studied. That's one option, but it relies on meeting all the monsters easily.
So instead, if you get an infocard for each monster you capture, capture more and each gives a % chance to unlock higher tier research. Otherwise you slowly learn more about the creature (the infocard adds information) and you just have a small chance of getting something useful out of it the more you invest into it.
Capture a spider, then designate an alchemists lab to it and you'll be more likely to gain "spider venom" for your weapons, designate a wizards tower and you're more likely to get "spider summoning" as a spell or "arachnids weakness" a racial bonus when fighting spiders. Either way you'll open up core demon summoning techs with any captured monster, each tier could require or be boosted by another number of creatures. Perhaps groups of monsters could unlock something interesting, rather than being too useful on their own. I used the infocard as a tool because you could easily have the infocard with the description obscured with mystic runes on the bottom to indicate that there's more to be gained, but the choice of what that is would be determined by your actions, blank for nothing to learn... something faint and almost hidden to show that it is still requiring more prerequisites

Still, what I picture as an option would be for you to have something like "Demon studies" - gives a demon binding spell, use the spell on a roving creature and bring it back to a city. opens up "Demonic existence" and perhaps eventually a single summoning spell "Demonic summoning". Assign a building to study your captured creatures and become a really interesting race of spider riding, venom spear wielding demonologists. Or perhaps you encounter more... lets say hellhounds... eventually with research summon fire elementals and get a boost to fire spells and attacks, or fire resistance. Encounter fairy troops in your area? If you're good perhaps the wording of the events could be different for good monsters... "You encountered a small group of delicate but powerful fairies, rather than harming them you bound them with magic." "You can: release them, kill them or take them back to your city in the hopes of learning to communicate with them and to learn their ways" The roles are reversed, but it's otherwise the same, someone playing in character wouldn't have an evil army of fairy troops... it wouldn't fit, they'd have to kill them, revive them and study the power of an undead fairy.

Just thoughts... I think some things are quite arbitrary... when they don't have to be, how would you learn about how to summon demons except by studies of demons? How would you improve your fire spells? study a fire elemental or just set fire to things, which would you imagine is quicker?

The same thoughts could be applied to necromancy... after unlocking it, perhaps you would have to kill a significant number of troops to really understand the magics of reanimating corpses of your enemies. Or sacrifice your own troops to understand raising the dead of your own race (to allow you to remain quiet and still grow your skeletal army)

Obviously all these things could be modded into the game in a superficial way... have an event trigger, pick a bonus afterwards. Use the event as a prerequisite for research... maybe generate a unit with no attack or visual range as a 'captured unit'... but it wouldn't have the pretty infocard bit or have been discussed and taken apart as well as is possible in the forums or by being in the main game. So... what's wrong with the idea? Limited by local monsters? could run out of monsters to find? could abuse a spawning point? would require too much micro? annoying?

-I'm not a good judge, I like playing x-com on hard and raiding for lots of ammo, having rocket launchers on day 1... or rather fighting against rocket launchers on day 1... and stunning poppers for a live exploding monster sample... psionic controlling anyone I can just for the fun of making them arm lots of grenades and drop or throw them around... and throw their weapons to you as well. (in elemental) If I could kill a dragon with 1000 troops or capture it with 50000, I'd capture it every single time. If I had to capture a live zombie... erm... a fully functional and highly bitey zombie... for research... I'd send wave after wave of people in to bring at least one back. Then find out how to control them in a useful way... or even to find out that they can never be controlled... equally interesting to me.

Perhaps an intersting thought could be that perhaps you could become stronger than the forces that were able to make these things in the first place. Maybe you could create a new race, with enough research... like the fallen were once men... perhaps you could make something that is greater than any fallen or better than any human. (either as a cosmetic change plus large combat bonus or, as in Fall from heaven, creating a new little village and switching control to this new race)

Lots of thoughts...