silva has palhares,falcao,santiago,stann,sonnen rematch,bisping and maybe lombard if he ever signs with the ufc. he has plenty.
gsp has diaz,story,ellnburger,macdonald,condit and rocha if he gets better stand up.
IMO in reality all that means is they both have an endless lineup of meaningless fringe top tenners where two new ones will sprout up for every one they defeat.
I don't think any of these fringe contenders have much hope of getting past the real top 5 contenders that have been tooled by the champ, let alone the champ himself. They bring nothing new that the champ hasn't already faced.
I mean what Diaz got taken out of the UFC by the likes Sanchez and Karo and has zero wrestling or TDD so will probably get taken down and tooled all night by GSP.
Take away his strikeforce belt, and he has no reason to be fighting a guy he has no chance beating. Diaz, Condit, Ellenberger, Story and Rocha would never get past the likes of Fitch, Shields, Koscheck or BJ let alone GSP. All of them have lost to those guys or someone at that level i.e. true top 5s. The only reason they're being considered is because GSP has cleaned out all other top WW fighters.
Same with Andy ... Santiago got tooled by the likes of Leben, Bisping and Palhares by Hendo etc. ... they offer zero challenge to the top contenders like Hendo/Okami/Sonnen let alone Andy. And Stann is just a slow brawler like Forrest, Leben and Cote that has no chance against Silva and brings nothing new to the table.
IMO The only matchup on your list that actually poses a threat to the champ is the Sonnen rematch. But with all the controversies surrounding that guy, it'll be a long time before something can be arranged again there if ever. Other than that, Okami is the last somewhat dangerous opponent left for Anderson, after that it's cleaned out unless they can bring over Lombard.
This superfight needs to happen, MW or catchweight. Otherwise it's obvious neither of these guys are in any danger to losing to the fringe contenders and will go to retirement fighting bored with one hand.