Bob Weir

@larsman 

Chops would not be an issue. Whether Payne could hang with the Dead's aesthetic is, for me, not nearly so clear. 

Of course, Garcia did manage to nab Nicky Hopkins for awhile. That would've been fun to see, in person. 

@stuartk 

Billy Payne did play with Phil & Friends on the Fall '99 and Summer 2000 tours.

The Dead had Bruce Hornsby in the guest seat for a time. His musical chops, ability to fit into the Dead's sound contributed to a brief musical renaissance for the band. Ultimately, he chose not to continue with them because of the vibe around Jerry's drug use.

@ezwind 

Well then, he clearly had no such reservations!

@wharfy 

Yes; I was aware of that but I've never heard any of those shows so I have no idea regarding Hornsby's stylistic versatility. 

Another great keyboard player: Reese Wynans. His work with SRV was stellar. He was for a brief time, the original Allman Brothers keyboard player but had to bow out to make room for Gregg.  

First of all, while Hornsby has stated that he was concerned about Garcia's drug use and health in general, I don't believe he's said that was the reason he "left" the Dead. Bruce was a pretty big star on his own at the time and it was always understood that he would just stay long enough to help with the transition to Vince Welnick on keyboards. He had his own career and band to get back to.

I'm not sure how Bruce would have fit with the band had he become the permanent keyboard player. He only played the grand piano with the Dead (and occasional accordian), so I'm not familiar with his othe keyboard/synth abilities. But I'm sure his chops are probably just fine. On piano, I found Bruce to be a more melodic player than Godchaux while Brent's forte was the organ. He also did a lot more soloing than other Dead keyboard players. Hornsby would often engage in extended jams into the drums/space segment with various combinations of band members. I remember one from MSG in '90 where he, Jerry and Vince did a gorgeous jam out of Fooish Heart:

https://www.youtube.com/watch?v=YOqKSpd_pv0