Let's start on a more fundamental level with the definition of "immediate impact," which I'll loosely define as receiving some meaningful snaps in crunch time of meaningful games.
Two further distinctions here: Early enrollees, and normal enrollees. As a general matter, it's going to be tough for even most of the early enrollees to have an immediate impact, and for the normal enrollees, it will be exceptionally difficult. It's very good news on our end, so not complaints at all, but there are just no glaring holes on the roster which will require a true freshman come in and be thrown into the fire like that, barring a bad rash or injuries.
Now... in terms of potential spots and specific players, safety and tailback are the most obvious spots. Offensive line is also a possibility. Wide receiver will also have a limited opening, but even that looks pretty tight. Scarbrough is certainly going to get time at tailback -- though arguably he's not really a 2015 signee -- but outside of him I think Harrison has a strong chance of early PT, as do Fitzpatrick and Ridley. I'd just about bet at least one of the OL signees makes it into the field and cracks the two-deep rotation, but it's too early to venture a guess on who that will be. Damien Harris would see some time at RB, I'm sure, if he were to sign with 'Bama.
For the most part, though, this isn't going to be a mass influx of freshmen getting serious time right away, barring the above-referenced unholy bout of injuries. That's not to criticize any of these guys, obviously, but there is a lot of talent returning on the roster with more experience, and those guys aren't going to roll over.
Will say this: All of you that think Barnett is coming in and winning the starting job, that's just not happening, folks. He might impress and the early enrollment will definitely help his cause moving forward, but he's not just going to blow by Coker, Bateman, and Cornwell. He probably won't have a viable shot at the starting job until 2016.