Actually pretty much all the questions will be answered next week. we had a bit of a revelation on narutaki's part in ep 30 in that he now understands that decade is not the destroyer that he made him out to be all this time, yes he may destroy but considering what happens next week its not all black and white as it seems and that from the looks of things narutaki may just be a disgruntled victim of the world's colliding. Also Kenzaki appearing suggests that the Rider worlds are fixing themselves, it may not be the case but I think its a pretty good theory when kenzaki says daishocker are not the true threat, and with him saying it has begun when the alt blade riders are erased.
As for questions pertaining to decade's past well, the movie has done that quite well from what we hear. The only thing that can really be dealt with atm is Daiki's involvement, we know he stole the dienddriver from daishocker but the whole sea cucumber thing was left untold.
