An interesting mechanic that might prove to not be worth it from a fun gameplay perspective is giving enemies the option to surrender or flee if they feal a fight is hopeless (just blew up all theirs friends with a meteor shower for example) ultimately though I think it would be frustrating to lose out on XP/ loot from enemies who fled.
Which is completely feasible if you give XP for "defeating" enemies (kill, flee, surrender, change sides) instead of just death.