I'd love to see -
Better battleline for Chaos. Less pillow-fisted chaos warriors, maybe chariot or warhounds battleline?
More options for Tzeentch spells, rule of one really screws over Fatesworn battalion and Horrors.
Also - and I have no idea why no one has mentioned this yet - a maximum number of special weapons in a unit. When it says some of the models carry X, some people take it as all of them carry it, and that's part of what makes Stormfiends so powerful. Perhaps as 1 model per X carries a Y.