There will never be a "Super Mario 64 2", at least as far as the name goes. The 64 name only mattered because it was on a system called the "Nintendo 64" and Nintendo was really pushing the 64 thing hard in those days because it was the only system that advantage. But nowadays, 64 bit stuff is outdated, obsolete, and laughable. Nintendo would never use that name for a sequel, even if it actually was a sequel.
Most would argue Sunshine and/or Galaxy are the sequel(s) and successors to 64.