I saw an earlier post and liked the idea of capturing heroes. This mechanic could work two ways:
a) Surrender: A surrender mechanic before a battle were a champion might be outnumbered/outgunned, and surrenders to the opposing forces. This could allow a further game mechanic, were you have the option to ransom back captured heirs (ala Game of Thrones...Jamie anyone?) and heroes. This could ALSO lead to other interesting mechanics, such as an option to force marry a hero to one of your heirs, or maybe have a 'evil ritual' were you sacrifice the champion to gain essence.
during combat: a skill (maybe learned?) or spell to knock out a champion with some sort of vulnerable status (ie bad morale, maybe really low hit points). Champion would be prisoner upon winning the battle.
The first question with suggestions are 'Will it be fun'. I think that it WOULD be....it could prevent losing powerful heroes who are jumped, allow for a 'forced peace' if you capture a rulers son/grandson, more diplomatic options, and adds flavor in and options in terms of gathering essence.