I think anything but a full blown reunion would be a step backwards for Slash.  Playing a few gigs like Duff is would be a step down and a poor idea IMO.

For me Slash has firmly established himself and his reputation post Guns now and to be associated with the current clown show in any way (short of an equal terms full on reunion) would reduce his cred and be a step backwards as far as I'm concerned.  Even a full reunion as Tejastech pointed out, the time has long past. Circa 2006 seems to have been the time performance wise, now Axl just seems to have lost his mojo, his fitness, his voice and his direction.  The only thing he has retained is his bitterness.

For a long time Axl was the ace in the deck that all the other guys were waiting on, these days at least for Slash that ain't the case.  Axl is more likely to need Slash at some point to keep his flagging career going rather than the other way around.