Stanley Clarke and Stuart Hamm would be one I'd love to see. And maybe Jonas Hellborg and Mark King (though I do have an MP3 of them playing a little slap improvisation together, 'm sure they could go much further than that!).
I once sat is a room alone with Billy Sheehan and Jeff Berlin as they jammed for about thirty minutes. They had never played together before that. It was kind of cool for me as I am a fan of both.