I'd say offer Robin a piece of the band name. I don't think he'd do that, but it would make Robin feel like it was more his band.

And we really don't know what Axl had to do to get him back the first time.

Did anyone get a bad feeling from Robin's "hippy rockstar" act during the last tour? That was totally out of character for him and it was like he was just playing a part... I don't know, it just struck me as odd. 

I don't think a reunion will happen in this case, because it will be an automatic admission of failure on Axl's part, and I think he'd spite himself by never performing again before going back to the other guys and admitting he couldn't handle it on his own without them.