Kabuto in Sword Art Online. That would be sick.

I don't feel like they'd make a whole lot of sense. Language barriers & live-vs-drawn being my main issues. I don't think anything else fits besides J-Dramas & I haven't seen enough.

G3-X & Robocop vs Terminator.
Blade & co. vs Royal Flush Gang.
Ryuki vs Mirror Master.
Kiva vs Morbius.

Kamen Rider W and Drive × Batman

Kamen Rider Den-O Crayon Shin-Chan, and Fourze Crayon Shin-Chan, both say Hi.

Drive X Saints Row because I think it would lead to crazy shenanigans :lol

I can't hear you over the sound of the entire Showa line up getting an animated comedy special.

Shelock Holmes & Watson x Kamen Rider W
Kamen Rider Drive x Hercule Poirot

So far I have:
-Kamen Rider Den-O × Doctor Who (because wibbly wobbly timey wimey... stuff.)
-Kamen Rider Blade × Yu-Gi-Oh 5Ds (because *ahem* Card games on motorcycles!!!)
-aforementioned W/Drive × Batman crossover
-Kamen Rider Kiva × Supernatural
-Kamen Rider Fourze × Space core from Portal 2 (Spaaaaaaaace kita!!!)
-Kamen Rider Wizard × Full Metal Alchemist
-Kamen Rider Gaim × Puella Magi Madoka Magica

I can't believe you guys haven't hit upon the one that's most likely to happen.

Kamen Rider x Precure All-Stars!

Even Decade wants to see it happen!