I'm 99.999% sure we've already seen what Grimlock's WfC design looks like, and he's definitely a design of his own. I don't think we've seen any in-game non-canon-fodder repaints of any of the designs other than in the multiplayer create-a-character feature.
I'm not surprised that Grimlock will be a tank in the War For Cybertron game, because that's what he was in the War Within comics.