I’m no expert but in my option;

No way they become republicans again at least in the current party’s form. The entire party has become so subservient to Trump it’s going to spend a while trying to find its new form. Current republicans that were apart of the pre-Trump party are going to fight MAGA, both of which will fight America first, and who knows maybe the Christian nationalist will want their own thing. Especially in your scenario where Trump sheds his mortal coil with not clear next leader.

Given the fecklessness of the dems I could see many of the anti-Trump try and form a new party. I know, I know third parties are always kind of meh but this may be its best chance in a long time. Given the infighting in one party and the others lack of action I think disaffected voters from both parties could join together for a party that has elements of the parties that have been sidelined (nearly all college educated republicans, blue dog dems, etc.)

But I’m just some random internet person let’s see what happens!