I can already hear you saying "Wow, how original, It's not like they are actively doing official crossovers or anything, noooo." Just trust me on this one ight?
So, most of the crossovers are pretty much just them fighting both of the show's bad guys. The problem is that there is literally nothing else noteworthy, except for some short reactions and extremely lazy powers. (I'm looking at you Gokaiger with the OOOs ranger key) Anyways what I'm saying is that make like an actual crossover, with actual relationships between kamen riders and super sentai (or power rangers).
This isn't really an Idea it's more of a plea (like all of my other fanfics). Also if you crave for an original character just make up a Kamen Rider power or smthn (Or just slap on the neo decadriver and call it a day)
(Unrelated)
Honestly i am really surprised at how many Kamen Riser x Symphogear fanfics there are on fanfiction.net. I honestly have 0 Idea on what symphogear is about but I listen to their songs on YouTube.