Back in the early days of the Ultimate Universe, it would always be fun to see how classic Marvel characters would get introduced in the new continuity. ("Oh, hey, Cable is future Wolverine!" "Look at that, Deadpool has an exposed brain!")
In recent years, that hasn't been nearly as much of a focus in the comics, probably because so many iconic characters have already shown up in the Ultimate books at this point. But since Ultimatum killed off so many famous faces, maybe it's time to go a little deeper on the bench.
The list of characters, especially ones from Spidey's side of the MU, who have yet to be Ultimized is not exactly A-list, though. Um, how about Hobie Brown, the Prowler? The mainstream version of the character is a frequent Spidey ally, and he'd fulfill the prevalent "black Spider-Man" rumors (as would another entry later in the list).
Then there's the rumor that it might be an Ultimate version of Miguel O'Hara, better known as Spider-Man 2099. As a Latino, it would similarly drive some attention due to much-needed superhero diversity, and the character has starred in a couple of recent Spidey video games last year's Shattered Dimensions and the upcoming Edge of Time, out this fall.