[Matrixbeast]

I don't think it'd quite work. Much like the SHT movies, there is a massive problem with clutter. They shove everyone in, and no one looks good because of it, making heroes who could take down armies look like weaklings and heroes who could barely save a Pringle from the can look like titans of the franchise.

If they did crossover Rider and Ultraman, I think they should start smaller, having it be [Current Ultraman who I don't know] with Gaim.
We'd have a focused cast without having to shoehorn in 30 years of characters, and the action could a lot better by actually featuring our protagonists instead of one-sided and dull fights that happen in about 15 seconds with no focus other than "Yeah. That just happened. Meanwhile, back at the plot..."