Veigar's Q
It's nice to have infinite AP from his Q but I'd rather have it have a cap. Around 150 cap but with a buff once you reach it. Maybe increase the range of his Q or/and W. Getting some MS maybe?
Could also make it a couple buffs per tiers. at 50 you gain a small buff. 100 giving another and 150 giving the last one.
EDIT: By capping his Q farm you can then empower his kit more.