[Joebe]

Hmm, reading back on the themes relating to the final form reminds me of something I tossed around in my head recently. When the protagonist acquires his final form, it was a point of divergence in the fictional history, one where he was able to overcome the challenge (physical and/or mental) and achieve control over his final form, and the other where he does not and gives in to the uncontrolled power (which could turn into one of them alternate timeline movies). In addition to the fossilized look of the form, the driver itself would be the one that's slowly fossilizing as he uses it, because the uncontrolled power the hero is suppressing with his will is slowly damaging the driver instead. When it comes to the end of course, there could be a final moment where the driver is nearly completely fossilized, and the hero may have to make a sacrifice to go into that last battle.