Melendez is fighting Sanchez, Thomson would be a great fight. The only issue with giving Pettis some huge fight is that whomever takes that fight could feel like they are in a loser situation. The 164 main card is somewhat stacked (Henderson/Grant, Mir/Barnett, Mendes/Guida, Koch/Poirer, Rothwell/Vera) with four big top ten fights and the vaya con dios fight with Vera/Rothwell. For all we know the only opening is on FX, now I don't think it will be an issue for Pettis, he's begging to get on this card. But I doubt any of the top 155lbers would be willing to fight on the prelims against a guy that they'll likely lose to and lose badly to.
Now if you put him up against some scrub you can just save the spot because you have to assume one of the six fighters in the two weight classes Pettis fights in are going to get injured and pull out.