[KRWolfblade]

The W/Double one would be interesting to see.

OOO, I think one of the games (Travelers Records) did something similar I believe with the original OOO.

Fourze's would be interesting to see since the whole Presenters thing was thrown in in the last quarter of the series and never touched on. A rider with the powers of all Zodiarts and the Horoscopes would be a bit over-powered unless he/she has a limit or can only use certain powers singularly (not combined).

I don't know how Gaim's would work but given how Knuckle Gaiden showed that some Sengoku Drivers still exist or are continued to be produced, could explain it.

Here would be some of mine:

Ryuki: Give an actually explanation as to how they all got their Rider forms/powers back despite the fact the whole of the series was erased from existence in the finale.

Den-O: A backstory/Gaiden for Kai maybe? Give a little more explanation as to why he helped the Imagin. Maybe a proper way to resolve Yuto's story?

Decade: An actual conclusion to it or an actual explanation of Narutaki