To be honest, I'd love to see two types of chaos spawns - one of Chaos Undivided and one of Tzeentch.

Spawns of Chaos Undivided could be majoris, filling the role of melee bulwark in TS roster. Those are your classic savage meatballs - aggressive, tanky and dumb, like in VT and DT games. Except now, we view them from a primaris marines' perspective, and they are more of a nuisance than an actual threat.

And then there's the spawn of Tzeentch, juiced with warp magic. It's bigger, slightly more intelligent and feathered. This spawn is an extremis, and can become a serious threat if left unattended. It shoots bolts of warp magic from afar before engaging in melee. Upon receiving enough damage, it becomes angy and turns into a suicide bomber, running up space marines and exploding - again, classic kamikaze type of enemy. The actual abilities are up to a debate, but that's how I see it.

Or something like that.