Tzeentch armies are good fun, not themost competive but sure stilll kick arse, groups of warriors with shield and hand weapon, with mark of tzeentch they got a 5+ parry save
Chaos knigths of tzeentch are the best knigths in my opinion, because you can give them the blasted standard to half the damage of any shooting + the six up ward they get from the mark.
you can either go for what we Tzeentch players call the unkillable sorcerer or
BsB taken on a deamonic steed to gain one toughness amd one wound, then you get him the third eye of tzeentch to reroll ones on ward saves, with the talisman of preservation that would be a 3+ ward save with rerolls of one.
I certainly love that.
for the rest of the army, maybe a hellcannon a spawn or two and the mutalith suits the Tzeentch theme.