[AkibaSilver]

If we're assuming the main riders can't make it, for whatever reason, then how about:
Ghost: Probably Spectre, followed by Necrom, althought I wouldn't say no to Onari or even Akari.
Drive: As much as I love my boy Chase, he wouldn't exist in an altered timeline, so how about Krim Steibelt, not dead? Gou also accepted.
Gaim: Takatora Kureshima is a god, and he needs to return. Failing that, Gaim has a billion rider. Surely one of them has a free schedule, right? Baron loves coming back for random stuff!
OOO: Date and Brotou. Make it happen Toei.
Decade: Literally any of the recurring cast. Yuusuke, Natsumi, even Kaitou.
Kiva: Seconding Otoya. I need to know how many robot puns they can make.
Den-O: Imagin pls. If that counts as Main Rider, then Kotarou please.