I don't think Diego should fight Matt simply for the fact that Matt may win. Don't take it the wrong way, I have nothing agaisnt Matt, it's just that if Diego losses, there goes another top contendar. I mean we can't see Hughes/GSP/Penn just keep fighting each other. This is what I suggest(many of these fights are already set, I know).
GSP vs Serra
Karo vs Fickett
Koscheck vs Joslin
Diaz vs Burkman
Diego vs Riggs
Fitch vs Menne/Fioravanti
Hughes vs Penn(in april or so)
GSP/Serra vs Riggs/Diego
Diaz/Burkman vs Koscheck/Joslin
Karo/Ficket vs (loser) GSP/Serra
Something along these lines would set up for many contendars rather then just Penn and Hughes. From there The winner of Diaz/Burkman vs Koscheck/Joslin would be a major threat, Fitch vs. Menne/Fioravanti would be a top contendar, Karo/Fickett vs GSP/Serra would either help someone get back on the winning track or bring some to the top, and of course the winner of Penn/Hughes would also get a tittle shot. The welterweight division has a lot of potential matchups and I think it would be smart to build up fighters rather then feed them to your top 3 or 4. You need to keep in mind that there are still guys I haven't even mentioned that can still be tossed around in this equation. I know my math may be a tad confusing, but the point I am trying to make is stuff only two or three contendars, give others a shot while keeping it fair and competitive.