[HIDE]Making them all canon doesn't really matter, as it doesn't truly affect the story telling and development of each series, and it's just fun to have homage. However, if I were to say what goes where:
Showa Era to Agito are all in the same universe (some in different parts of Japan). Shared villains and crossovers that are still in continuity explain Showa Era to J, with Kuuga and Agito able to exist in the same universe as they neither deny nor contradict what was established before them. Agito is pretty much a direct tie in to Kuuga with G3 and other nods to Kuuga.
Ryuki doesn't matter and can be rewritten to work no matter what, since the original series reset itself into ret-con of the Mirror Kamen Riders no longer existing with who was established as them.
Faiz, again, could be in the same universe as the others since the show neither denies nor contradicts what has already been established. Smart Brain could very well be an offshoot of one of the original evil organizations who discovered a way to more easily transform man into advanced beings when they discovered the Orphnoc King and the belts.
Blade could also be in the same universe, with the Dark Roach army possibly being contained in whatever city they were in. This is unlikely, however, as the origin of species may mess up previously established history. It’s always possible that scientists got it wrong before, that’s not too farfetched.
Hibiki probably shouldn’t be considered canon, though it wouldn’t be impossible to link it as it, too, does not deny nor contradict other Rider tropes.
Kabuto probably should not be canon, but it’s shown that they can rewrite history with the Hyper Zecter, so like Ryuki, it has a ret-con device that could make it work in itself.
Den-O, they travel in time and have already meddled in history a lot, so they can go wherever they want.
Kiva could very easily be established in Rider canon, with the IXA system being developed between the end of Black RX and everything else. Fangire appear to be an isolated incident, with the only member who ventured out being Rook to exterminate the competition. IXA could very well have been inspired by good guy Rider development like X and Super-1, and the Kiva system may even be inspired by Amazon “Incan science”.
Decade, of course, is where a lot of things get convoluted, but it established the existence of parallel Rider worlds (just look at Black and Black RX), so Decade can go wherever the hell he pleases.
W is the first Rider series to invent its own city and back story to go with it, and the name Kamen Rider was given to W by the people. This is very easy to shove into Rider continuity.
There’s two OOO universes. And sadly, the OOO series is not in canon with everyone else. The OOO movies are in a different continuity from the series, but exist in the same continuity as the rest of Riderverse. So Kamen Rider Core is canon. ****.
Fourze exists with the rest of the Riderverse for now, unless it contradicts everything that’s been established. Until we find that out, it can stay. If it changes, then Fourze just created an entirely new Rider World for Decade to **** up.
Any major events that should have been reported to the general public outside of the city they occurred in, such as the Neo Fangire appearing from the future, can easily be explained with government cover-up or Rider ret-conning with time travel.
Enjoy mind ****.[/HIDE]