FFR was ridiculous, but just passable enough because they all had some sort of gimmick thingy which they could turn into, though I still think it would have been cooler if they'd just summoned the real thing. Like, rather than having Kuuga turn into Gouram, why not just have Gouram show up for them to both ride on? Or maybe it just changes them to their actual Final Forms? Though then you don't leave Complete Form much to do, I suppose.
With the older Riders though, it'd be harder since there's less stuff to choose from. Turning into their bikes is just redundant, so what's left? V3 becomes a giant V3 Hopper and falls over, crushing enemies? Super-1 becomes a giant Power Hand fist and pounds them into the ground? Riderman becomes a giant Rope Arm and throws them hella far? Amazon becomes a giant version of his own head and bites the monster in two? Rider 2 becomes a giant can of Whup-Ass?
Actually, I better stop there, I'm actually starting to like this idea.