When you're enemies with another faction and wish to "defeat" them, you have one option: Murder them. Obliterate them. Remove them from this mortal coil etc, etc. War is never that simple, be it real or fantasy. So why not have the option to take them hostage?
I've been a HUGE fan of the Sega Saturn/PS2 game Dragonforce since the mid-90's... if you have ever played this incredible masterpiece of a game (and if you haven't, I thoroughly recommend you do), I believe the enemy-capture element in Dragonforce is something that would fit perfectly in Elemental (if not an even deeper version of it). After defeating more "important" units (sovereigns, and other strong yet unique-named humanoid NPC's etc), you may get the option to take them as prisoners instead of simply killing them on the spot. I think the option should arise when you give your unique foe a near-fatal blow. Overly-powerful one-hit kills are too wreakless to warrant such an opportunity. At this point, depending on how the specific faction viewed you and numerous other variables, I could see a wide number of possibilities should arise to keep things interesting:
- The captive(s) would rather die than be hold captive to a tyrant like you. Just when you thought the battle was over, your potentially spared opponent sees an opening to attack you, inflicting on last blow. Whether they manage to hurt/kill you or not, some simply like to go out with a bang.
- They suddenly make an escape immediately after you decide not to kill them. This likelyhood could be dependent many issues; If your move rate is significantly lower than theirs. It could be dependent on how many units you currently have in your party; having only one unit would make it difficult to take any captives, especially multiple captives. Also if your units at the end of the battle are weak, this drastically raises the possibility of an escape. The escapee may retreat to another town in their own faction to plan their revenge, a town belonging to another friendly faction, simply roam the land alone and injured, or vanish from the world completely. No matter the outcome here, this is going to make the player seriously regret not ending their life on the spot.
- You get lucky and they decide to immediately join your faction on the spot, making them immediately playable after capture. This may or may not be a good idea, as these new recruits won't always stay loyal. A good indicator of this would be if their former faction still exists and if it's still strong, they may try to go back to it.
- The prisoner tells you they don't wish to join you and your faction. You now have the option to execute them at any time, or keep them as prisoners to see if staying locked-up for awhile doesn't change their mind. Feeling guilty? Clear your conscience by simply letting them free (although if they still have it in them, this could result in getting one of your own units getting killed on the way out). Bring up a menu to visit the prison any time to check on their status and feelings towards you (and of course if you have the sudden urge to execute them). Perhaps once you've exterminated their entire faction, figuring there is nowhere else to go, then they will freely join you. Some prisoners may be bold enough to ask you for bribes first in order to join you. Bribes may be a sum of gold, or even a quest, like hunting down an old foe, or tracking down a friend or lover from their former faction in order to reunite them. Find that person, and now you've brought two new assets into your faction. And of course, some prisoners will simply never join you, or god forbid, rot to death if you wait through enough seasons.
- The enemy faction may attack your city for the sole purpose of rescuing their former high-ranking generals. Be leery if you see an army coming at you of the same faction as your prisoner(s).
- Feeling evil? Well good thing you're a channeler; you also have the ability to use new mind-altering spells to help "convince" captives to join your twisted cause. You won't see this ability until you reach a very high level of magic, you need very high intelligence to cast it properly, and it'll take an enchantment slot for each prisoner who's... altered. If the spell fails, the captive's brain will fry... what a waste.
- Feeling evil but not so adept at spells? Sell your troublesome prisoner into slavery. You lose the prisoner, but receive gold and a steady increase of materials for the coming years in return.
- While any of the above situations are transpiring, other formerly-affiliated or even un-related factions could react negatively or positively due to big-name hostages (putting a well-known sovereign into slavery is sure to get someone's attention). This could earn you new friends, enemies, a new war, or simple requests to let someone go, transfer the prisoner into their custody, or they may simply want you to execute them immediately. You may also end up with a massive reward for helping fill these requests.
Another interesting aspect to this is the prisons/dungeons themselves. Since you're always building in Elemental, this presents a great opportunity for a new building (and certainly one of the most useful ones) which you can create once a town is large enough (perhaps level 2 or 3). After you capture any unit, they remain in your party as an unplayable character until you drop them into the nearest jail. Keeping them in your party for long periods of time significantly raises the odds of an escape, or even an attack. Therefore quickly getting into a town with a prison is vital, if you truly care to keep them around. I don't believe escape should ever occur once locked into their cells as long as the town itself is secure (especially when there is a unit or two stationed in the town); it should be something the player can always rely on being 100% functional. However, if the town is overtaken by another faction or group of creatures, all prisoners in that town could be released outside of your control, turning them into factionless renegades. You may even find yourself having to transport prisoners to another town if the current one is in serious danger.
Lastly, I don't believe capturing has to be something that only happens at the end of a battle either. Something I crave more in this game is diplomatic options. While you can "trade" certain aspects of rival factions like buying specific units, I can also see capturing/assimilating being used if one faction is vastly more powerful than the other. In other words, instead of doing the same song and dance by always battling to get what you want, you can now negotiate with other sovereigns who may completely hate you, but would rather hand themselves over to you peacefully than allow themselves and all of their people die in pointless bloodshed. This way you can take control of not only entire cities and resources, but all of their troops, and their sovereigns, without ever getting into a fight. Taking this concept one step further, an option to make a city or even entire faction as a semi-functional "puppet" (ala Civ5) would also be a great addition.
Okay, so this went on WAY too long, but this is something I truly believe will bring Elemental to the next level. For a game that blatantly revolves around war, the concept of capturing your enemies with deep, complex gameplay elements is an absolute no-brainer IMO. I have no doubt these features will leave players feel they're playing a far more complete, enjoyable, more intelligent game.
Please let me know what you think/like/dislike/want to flesh-out/add to the game (especially you devs), and thanks for reading my endless rambling. 
