I think Sakurai's take is that third party characters not appearing on Nintendo systems aren't banned, but that Sakurai is still reluctant to include them. Sakurai loves games from all systems, but at the end of the day he still has to answer back to Nintendo and he still has to answer to the other company involved.
Even if Sakurai is up for it, it's still up to Nintendo and the other company to cooperate. So if he wants, say, Master Chief, it's still up to the pair of direct competitors to decide if it's going to tip the balance unfairly to one or the other somehow. Someone on one side or the other may not be OK with the involvement.
And to be honest, outside of that specific criteria, there aren't very many characters who fall under the criteria of never being on a Nintendo system.
Pretty much the only roaming major developer I've found so far that doesn't show up is Valve.
You want Crash? Lara Croft? Commander Shepard? All already there.