Ultimately, I think if the Star Wars and Star Trek universes were to ever converge, Hollywood would probably collapse and be sucked into some black hole.
But the Federation would be a powerful ally for the Republic (or Rebel Alliance), although I suspect the Romulans would side with the Empire. Jabba the Hutt may have some competition with the Ferengi moving in on his territory looking for business opportunities.
And of course, the Trek universe has the Q Continuum, the Douwd, the Organians, and other powerful species which make The Force seem like some kind of cheap tractor beam.