I think we're forgetting a key component for Springfield's potential replacement: Yes, of course we're looking to see what would be the most marketable franchise to replace Fast Food Blvd...but more importantly will sell the most in food sales.
Yes, Wicked would be a potential candidate, and you could argue it would sell well with merch, but I'm sure how you can really push the food with Wicked. The whole reason Springfield came to be in the first place was because the fictional food was a big part of the franchise. Yeah, I understand they already had the rights to it in the first place with the ride already there, which was a huge driving factor, but while I'm not privy to the exact numbers from the food sales of SF, I imagine they make good money from the food. A big selling point of Springfield was "Oh, I get to eat at Krusty Burger, drink a beer at Moe's, and get a donut at Lard Lad's!" If I were a bean counter at Universal, I would be looking to see which IP is known for its fictional food to at least match those food sales Springfield has. The food at Springfield is an even bigger draw than the ride is, and I imagine they would prefer an IP that could have the same draw.
Think of some IPs that would have you go "Oh yeah I would pay serious money to eat there!" Spongebob is absolutely one of them. Maaaaaybe Shrek with Friar Bob's and Donkey's (Chicken and) Waffles.
EDIT: I posted this thinking this was the Florida thread, so I'll post this over there as well. The same line of thinking should still apply for Hollywood though.