ToME: the Tales of Maj'Eyal

Partly because I like the level wide effects and partly because there should be a better warning about using teleportation there:

In addition to some sort of fire/arcane/magic gone haywire effect, it could also mention that teleport spells are deflected.

A magic gone haywire effect would make it more difficult for the classes that are already most hurt by the teleport effect. I'd rather see that go away at least for LoS teleports, but if it is to stay it should have an icon like the high peak zone effects.

Well for a magic gone haywire effect, I do something like reduce spellpower but increased spell crit.
Any effect doesn't have to be huge or detrimental.

Since the sorcerers are pretty much turning the whole place into a arcane-sucking/channelling nexus, I could see something like increased mana costs but also more crit/multiplier or shorter spell cooldowns. This would make for interesting interplay with the "Can't rest God will eat me" element of the stage since you're blowing things up easier but also have to jealously manage your mana.

It'd be cool but tricky and potentially frustrating to give spells weird random effect/misfire chances too. Probably too much chance for random near-end-game screw though.

I could easily see haywire magic having a negative effect on antimagic shield.