They're using other characters as replacement since we don't have F4 yet.
Groot = Mr. Fantastic
Cull Obsidian = The Thing
Frost Beast = Invisible Woman
Extremis = Human Torch
Now for the season finale, at the very least Rhodey has to suit up. Or else how the hell are they winning against a Super Skrull
That would also help him "redeem" himself for Armor Wars (he's clearly not evil, but is serving as a sort of antagonist going against Nick), setting up him against the government using Tony's tech.
Only other hero I can see showing up could be Monica, and that's a big maybe, considering the finale of WandaVision and how this could lead up to The Marvels.