I'd have to say I considered Springsteen for about 2 seconds and came back to my original conclusion. While Springsteen may be the greatest live act ever, its still wouldn't be right. Paul needs to perform alone. Maybe doing a song w/ Bruce would be great but I would leave it at that.
Although I just saw Bruce a few weeks back and even at 60 he puts on a show that puts young bands to shame. 3 hours of non-stop high energy rock, requests from the audience, interacting with the audience. There has never been anyone like Bruce and there never will again. But still after all that praise he can't hold a candle to Paul. But if anyone had half a chance it would be him.
Plus, can you imagine if they both did their full set.. we're talking over 6 hours. If I'm at a concert for 6 hours I want it to be all Paulie
PS- Chris, best post ever