I don't think Maia nor Leben would stand much chance against GSP. Maia just couldn't get him down and Leben is just junk compared to GSP in every aspect.
Of the names you have mentioend, Bisping definately has the best chance, but he cut from LHW to MW and I don't think he could lose another 15lbs.
What if Johnson beats Marquadt though? He would then have some good wins to be in line for the title and surely has the tools to give GSP a very good challenge.....
Diaz and Condit are the only other challenges I see for GSP at this point at Welter. But that is 3 potentials right there, enough to keep him busy while some of the younger guys develop.