I'm on the fence about that kind of thing. We know that the original actor for Kuuga is just not going to come back. I'd hate to see Zi-O time travel to an age where Decade was active, just for the sake of using all the recast Riders (like Decade's Kuuga/Onodera Yusuke), but it may be the only option we can get in some places if the TV show wants to gives some focus time to EVERY Heisei Rider. Granted not all the recasts were bad, I actually like Decade's Souji/Kabuto better than the original, that one actually had a likable personality. But I think the long-term audience would be happy if they kept recasts to a bare minimum. If they HAVE to recast previous Riders, then I think Toei should forget about the ones that appeared in Decade, and recast the Riders all over again with new actors. Like I said before, they could enforce the idea that "the mere act of time travel changes things", as in time travel is a taboo thing that shouldn't ever be done, and it's making the past of the Kamen Riders a little unstable. In other words, Doc Brown logic, alternate timeline tangents, etc. At the very least, it might provide a good reason to have a Zi-O VS Den-O moment. And let's face it, with Kamen Rider doing the time travel gimmick *AGAIN*, it's bound to happen.