And the question in the OP is too hard to just narrow down to one. I can do one per position group though.
QB: Dak is already a probowler but he can still emerge into the elite top 5 category if he improves his downfield passing. He starts making consistent big plays and he moves up another notch to All Pro. We have the running game to give him the looks necessary through play action.
RB: Keith Smith showed that he could dig out LB better than anyone we have had since Moose calls filled Texas Stadium. I love watching him on the goalline. His 40 time is awful at 4.8 but he does have the agility and change of direction to run routes. If he can learn that side of the game a pro bowl is not out of the question given the paucity of FB in the NFL.
WR: We really missed Dwayne Harris when we let him go. He has not been the difference maker in NY that he was here thankfully but Beasley, Lucky, et al have been uninspiring in replacement. Switzer was electric and college and if he can emerge in the same way for the Cowboys then the kicking game gets the biggest boost possible.
Listening to Dooley talk about putting him and Beasley on the field and replacing them with DBs has me salivating. I like the idea of out OL pulling and pancaking nickelbacks more than a little. Then there is the play action off of that. Switzer needs to emerge as a threat to make that happen. Matching him up against LB make it a real possibility. The LB corps in the NFCE are particularly uninspiring.
OL: Collins bit as a puppy. Last season I could see the strength he added but I could not get a feel for his awareness in pass protection. That was the weakest part of his game. Him being able to come in and play at a high level is critical to maintaining the status quo. He also has the agility that Leary lacked and so improve the SQ.
TE: Hearing that Witten has taken to Rico was a breath of fresh air for this fan. Rico is not going to outrun people and is going to have to leverage his size and strength to succeed on routes. No one runs stronger routes that Witt in the league. Good man to learn from.
Similarly he needs to learn the steps and hand placement to block inline and out in space. It's a tall order but his ceiling is as the best blocking TE in the league. He has the size, strength, and quickness to pull it off.
DE: Taco is raw. He doesn't have consistent technique in his pass rush moves. The only move that looks polished is his spin move and he often goes back to 3-4 techniques where he stacks onto the OT and paddy cakes trying to rush the passer.
Similarly he would surrender contain or allow himself to get sealed by trap blocks on penetration by smaller players. Not surprising for a guy with one year rushing upfield but he has a long way to go. Marinelli likely will be all over him demanding consistency in the technique he is demanding but how much technique he can pick up next year will go a long way towards how successful the DL will be.
In the playoff last year we only had three effective rushers as DLaw, McClain, and both Crawford's play went into the toilet. I get that they were injured but the poor play is poor play. We need more production than just Irving, Collins, and Mayowa.
DT: I saw Irving do his best work inside. I get that he stuffed stats beating up Cinci's RT but he was a turnstile that made Free look good in comparison. Irving keeps flashing but needs to take that next step to consistency. He has All Pro potential but needs to realize.
Reports from Derek Eagleton that he has put on 15 lbs of muscle are encouraging.
LB: Jaylon Smith. I'm convinced in a full recovery now that the outer extremities of the peroneal are innervating. Goodness knows we battled over it this Winter and Spring.
Even with a full recovery though he will have been away from football for 18 months. That is a tall order to overcome for even an elite talent such as he.
OTOH, he has the speed, size, and quickness to cover any offensive receiver they want to put against him, he has the most pass rush skills of any LB on the roster, and the size and quickness to take on the run. Coverage in the middle of the field was our achilles heel up until last year where we gave as good as we got. With Jaylon on the field we could lock it down. I drool over the thought of he and Lee sugaring those A gaps.
CB: The rookies are going to have growing pains and we are turning over at least 3 spots in the DB rotation. Scandrick struggled coming back from injury last year suffering from hamstring issues all season. We need veteran experience in this system. He has a major fire lit under his butt and the chip on his shoulder to make it mean something but we need him to comeback strong particularly early in the year.
S: Byron Jones finally settled in at FS and teams could not throw over the top and the corners played much better with competent help provided over the top.
His man to man coverage was pretty good except in the red zone where teams isolated him on TE for TD throughout the year. Compounding matters was his inconsistency coming up in run support. He was not bad at anything outside of the RZ woes but if he can put it all together it would go a long way to galvanize the secondary and coverage units.