Obviously, Jameis is Jameis. I have concerns over Mariota, he needs development. But if he comes back and shows improvement, that'll make a big difference as he's extremely athletic with a strong arm. Bryce Petty is another guy with a lot of potential due to his size, athleticism, and arm strength but he's old(going to be 24 I think). Probably should have come out this year. Brett Hundley is also athletic with a big arm...but like Mariota, he needs development and needs to improve in a lot of areas in terms of consistency. Those are more the first tier guys.
Devin Gardner is intriguing as a second tier guy. Still needs quite a bit of development but shows some NFL caliber attributes. I haven't seen Chuckie Keeton play but I heard good things about him. We'll see how he comes back from his knee injury. I don't like Kevin Hogan of Stanford or Sean Mannion of Oregon State because both have terrible throwing motions. Both have very slow, weird, and deliberate motions. Jacob Coker of Alabama is intriguing because he has size(6'5" 230), mobility, and a strong arm but he hasn't played yet so we'll see about him. Could develop into a MAJOR prospect. Connor Cook of Michigan State shows promise too but I don't see him declaring in 2015.
There are also guys like Gunner Kiel, Trevor Knight, Jared Goff, or Jacoby Brissett who could declare with big years and come out of "nowhere" to an extent.