Any other CA cichlids potentially...SA's don't mix well but CA's (from what I have read and been told) aren't all that fussy...
The only thing for me with hybrids, like mentioned above, is an inability to get fry/juvies passed on which means the tank owner has to either get more tanks for fish numbers or kill fish...the second of the two seems a bit unfair if a concious decision is made to go this route...that said if someone can breed something really nice I would be at least a bit interested
edit: Blue Convict / Yellow Convict might be cool...and wouldn't upset too many people. You may end up with a smaller multi-coloured convict of sorts...mixing any of the Archocentrus family might not be such a bad thing:
www.aquahobby.com/zeco/genero.php?gen=Archocentrus