I’m not sure there are many other franchises that could crossover with Monster Rancher as well as Ultra Kaiju/Man/Q did
Few I can think of have monsters able to frequently be benevolent or nonevil, not have a bunch of complicated pokemon like evolutions, or allow a lot of creative freedom in altering color schemes
You so much as change Godzilla’s color in a videogame without permission and Toho gets mad. Everything must be on model.
And I’m not sure monster Rancher would be willing to make you an evil protagonist breeding monsters to sell to villains to use as part of a monster army.