another fun trick with tzeentch is to look at the rules for the chosen of ahriman! granted you'll need opponent's permision and they're not usually tourny legal, but it's simply good wholesome fun teleporting entire squads of rubric marines around the table!
(well, it's fun for the chaos player at least...)
you might also want to look at getting a defiler as they can really help out in the anti-meq department, since your marines compleatly lack any form of weapon upgrades...
units of flamers can make a mess of anything not wearing power armour or better too! (and all told, are fairly cheap pts-wise too!!!)
tzeentch lords can be just as killy as most other chaos lords! i like the bedlum staff myself - it's like a thunder hammer on steriods! (though it does lack the high S of a real thunder hammer...)
however, striking in I order and the nasty side effects of getting whacked by it are pretty nice for hunting big nasty things and enemy characters... plus, with d.strength you'll still wound most things on a 3 so it's still tasty! (you can get S6 if you go for stature...)
cheers!