…what would it be and how?
I would do a complete overhaul of the Magus. You see the problem with Magus in WAR is they’re nothing like the tabletop and background story version. In Warhammer the Magus (and in fact all Chaos Sorcerers) are heavily armoured; they wear Chaos armour, which is actually the same as Chosen. Also traditionally the Magus has very powerful nukes, but they are only short to medium range. And to top it off, the Magus is also not too shabby in melee combat.
So essentially you have a heavily armoured, short ranged nuking, power house. Which is exactly what I would have made the Magus in WAR. OK I wouldn’t make them as tough as a Chosen, but I would have made them significantly tougher than traditional casters. After all, they would need to be to stand a chance of performing their medium range nuking role. Then I’d try to make their spells actually reflect those from the lore, so it’s bye, bye DoTs and hello huge nukes and uber debuffs.
Just because MMOs normally give every faction a long range nuker, it doesn’t mean they must have one. Attacking Keep walls a problem? Well sorry this is Chaos, their whole theme is meant to be melee and short range. Plus that’s what their War-machines are for. If the ones in WAR were good enough I guess. Hellcannons are such a disappointment.
Oh, and no more summoning Nurglings, that’s just sodding silly.