'Megamagic' for Multiplayer: Channeler's Death Curse
I'm still leaning to having single-player games end if you lose your channeler unit, and I think the idea is even more appropriate for multiplayer. If we do end up with dead channeler = game over for you, in multiplayer, I'd like to see something close to a MAD option where a channeler who has studied appropriate magic can use his or her dying breath to lay some seriously bad vibes on the channeler whose forces have dealt the deathblow, or perhaps on whoever you think is your greatest enemy.
Maybe a Death Curse would be simply an intense version of one of the landwrecking spells centered on the freshly-dead channeler's body. Or maybe Death Curses could somehow reflect the channelers' identities and histories a bit--say a slain good channeler being able to do a mega Turn Undead against his necromancer archenemy, forcing her to start from scratch with her undead forces.
My fuzzy idea here is basically to work up a magical corollary to the more traditional trick of trying to help your enemy's enemies when you know the game is over for you.