Tri Las preds have won games for me before - I'd keep them
To go bigger, would you be averse to more khorn?
A juggernought Lord and a unit of spawn are nasty.
You could bring a tzeentch Demon Prince with the black mace.
I'm not a fan of helbrutes with autocannons - they just don't do enough. I swapped mine's arm for a multi melta and keep him in reserve so he doesn't give away first blood.
Edit:I love oblits. I take them in every game almost