A possible alternative to killing off opposing heroes...

I think that it should work something like this. you knock out the hero by bringing his health down to 0 and it goes to the reward screen. then during the reward screen there would be a small chance that it would say that you have crippled the enemy hero and that he has asked for your mercy.

If you spare him then he would join your side and fight for your cause after 100 days of rest in your capital city. (after all he is crippled) 

He would get to keep all of the equipment  that was on him and his upkeep would be equal to what you would normally pay one of your hero's that was at the same level. 

If however you choose not to spare him then he would die and you would get to loot 1 epic quality item from his body, you would then be given a box with a list of 5 random items to choose from. The item should be of a slightly higher quality than what he would have had on him if you had let him live

what do you think about that ?

Or there could be a faction trait for Altar that allows them to hire any hero. That would give them a better hero advantage and really make them unique. 

Let you hire them, but give them a small chance of defecting (or refusing to fight) every time you send them into battle against their native faction?

I like the system as it is. I wouldn't mind be able to recruit opposing Champions but it should not be a matter of just having some extra Gildar inyour pocket. More like: "Let me research some more and I'll come back for you... if you are still here."

But still be able to kill the Champions? Yes! They are a limited resource that can be destroyed (if opposing, so the enemy cannot recruit him) or used (if ally, so you can use it for your own benefit or deny him to factions of your same aligment).

One solution could be to add a technology to the magic tree that allows for the recruiting of opposite alligned heros.  Maybe in between the tier 3 and tier 4 recuriting techs.

I'm definitely for some kind of permanent death for heroes. Right now, they look immortal. Lost tactical combat should not always result in death, but there should be a chance to have that. Let's say one of a 10 injuries. You know the game blood bowl ? It's a roll between a list of injuries and one entry is death.

The problem is there are now a finite number of heroes at the beginning of a game, that's why they need to be immortal. Some other posts have suggestions about how the game should have new heroes (dynasty would be my favourite obviously, but spells, random chance based on prestige, quests rewards would be great)

What if there was a chance new heroes could be spawned by army units reaching a certain level?  So your army reaches level 10 with 200HP or whatever, and you're given the choice to trade it in for a single lvl 1 hero with 20HP but a bright future.

Text could be something like "A true champion has emerged.  Through the fires of battle, a hero has distinguished himself from his peers with amazing acts of bravery/cunning/whatever (depending on hero type being offered).  Do you want to leave this leader amongst his unit (no change), or single him out for further training (gain hero unit, army unit returns to lvl 1).

That way, you'd always be able to get new heroes, so it's more workable to kill off heroes if they're defeated.  And there's also a significant trade-off to taking the new hero.

As long as the AI also kills off opposing champions then I don't have a problem the way it is. I've seen wandering monsters kill them off - which is pretty cool that the world is also playing against itself. Makes the initial rush important and makes getting a scout important early on.

Just made a suggestion regarding choices and reputations but this could be a good option for handling the assassination of the champions.

What if, as an example, you are an empire player and that for each unclaimed kingdom champion you kill off, the level of that champion becomes a negative reputation modifier for all the other kingdom factions. Kill off enough and they will hate you very quickly.

I'm okay with professions providing this sort of option, they should really be more game-changing than they are.. but that is an aside. The ability to kill enemy heroes should stay, but actually killing them shouldn't be so incredibly easy. They probably also shouldn't drop epic weaponry/equipment.

Someone mentioned giving them bodyguards, and I completely agree with this. After all, how have they managed to survive for so long, and why would they be hesitant to join your cause? The bodyguards can go away afterwards - this doesn't have to change the balance of hiring heroes - it is simply a measure to keep the high tech requiring heroes around long enough for the Player/AI to research the tech to hire them.