I don't know if I am jumping the gun a bit with some magic ideas or if they have already been added and I don't know, but I figure the earlier the ideas get proposed the less annoying they are to add them if they pass muster. Anyway, on with the proposal.
I have always enjoyed the idea of, when major magical powers fight, its not just hundreds of soldiers dying on either side, but entire citys are destroyed and, in some cases, whole regions/continents are blasted to wasteland. Perhaps, when a channeler owns whole contenints and has vast powers, and he goes to war with another channeler of similar power, they don't just throw metor showers and fireballs and artic blasts, but instead call down real asteroids the size of texas to level entire citys and leave chunks of land as a blasted hellscape. Spells that create spreading viruses that make the black plauge look like a joke and slaughter not only troops but citzens, crops, animals and drain the life right out of the ground.
These sorts of spells should be the equivelent of ICBMs from CIV. Very high up on the tech tree, hard to make, difficult to use and having/using them is a big diplomatic hit. But when used entire races get annihilated.
Another option could be creating a single zombie. Not a zombie like from most fantasy games, but a zombie from one of those B- horror movies that infects others with its bite. Cast a massive, difficult spell to create a single zombie in an enemy nation. But unless the other player acts quickly to kill the zombie, the number of zombies increases exponentially. If the zombie hordes are not dealt with an entire continent could be lost to the zombie.
Once my channeler is immensly powerful and capable of amazing feats of magic, I would like him to have the ability to cast some truly horrifying spells that lay waste to huge areas. MoM had a couple spells like this. One caused 2-3 volcanoes to randomly erupt anywhere in the world each turn. The other caused every soldier lost in battle to return as a zombie under your command.
In a lot of backgrounds for fantasy settings, there is often a massive dessert/wasteland that is attributed to a fight between two wizards thousands of years ago the destroyed thier nations. I want the chance to be one of those wizards and really mess the world up when I start casting war spells.
Anyway, thats my 2 cents.