Good question. As the roster becomes more stable, the opportunity for "step-ups" from an unexpected player diminish.
I'd put Broxton at the top of the list. He's not the closer that he was in his youth with the Dodgers and he won't dominate like Chapman. It appears the job is his to lose for as long as Chapman can handle the rotation-hopefully all season.
Mesoraco is at the top of the list too. Hanigan is a good receiver, but they probably don't want to rely on him for any more action than he gave them in 2012. Mes's lousy rookie season doesn't worry me much. He'll be 25 next year and catching at the major league level is very difficult for most rookies. I always remember that Mike Schmidt failed to hit .200 his first season with the Phillies. I'm not hanging any Schmidt-like expectations on Mesoraco, but it's not far-fetched to think he can have a good season splitting time behind the plate. He's seven years younger than Hanigan and I'm sure the Reds still expect him to eventually take over the lion's share of the duty.
Chapman could be on the step-up list for a second consecutive season if he has any semblance of his 2012 effectiveness spread out over more innings.
Defensively the biggest opportunity for a step-up is Choo's glove in CF. He played there a little early in his professional career while he was still in the lower levels of the minors in the Mariners organization, but it's pretty much a new and more demanding defensive position for him. Offensively, everyone is just hoping he'll keep doing what he's been doing at the top of the lineup.
Masset could be on the list. Right now we're plenty deep without him, but if he has a successful return from his injury he has the stuff to force his way back into a prominent role in the pen.
Injuries often provide step-up opportunities-ala Frazier last season. Hopefully we won't have many of them this time around.
Right now I don't see a spot for Leake on the staff, however, we were probably very lucky to avoid the injury bug in the rotation last season. It's probably a bit much to expect two consecutive years without a single starter missing a turn. Gotta hand it to Leake, he's already stepped up a couple of times in his brief career. No one expected him to be in the rotation after they drafted him in 09 without throwing a single pitch in the minors and he turned out to be their most consistent pitcher over the first half of the 2010 season. Early injuries to other pitchers returned him to the rotation in 2011 and he turned out to be the best starter on the team outside of Cueto's shortened season. His 2012 was nothing to write home about, but the Reds were glad to get it at the back end of the rotation, especially at a minimal impact to the payroll. Right now I don't see where his opportunity will come from in 2013, but if it materializes he's capable of answering the call.
I wouldn't be surprised to see Cozart take the next step in his development. He's following the typical rookie path: Start out hot then struggle when opponents learn how to pitch you. The good ones make adjustments to what they're doing and the others continue to struggle. Cozart stung the ball in a cup of coffee with the Reds before his injury in 2011. He started off well last April before his average dropped. Overall the Reds have to be happy with his rookie season. Now we'll see if he can avoid a sophomore jinx and take the next step. He could stand some improvement to his plate discipline, but that's nothing unusual for a rookie. I don't ever see him as a player with a huge AVG/OB% differential, but he should be able to lower his K's down to double digits. Besides, now that we can move him to the bottom of the lineup getting 15 HR and a .250 AVG from a middle infielder with a solid glove is an asset. If his OB is a little low it doesn't hurt as much as it did when he was batting at the top. Also, it's easy to imagine him cracking 20 HR in GABP.