I think you will see improvement in a few players:
Roth- with a year under his belt at OLB will be a very pleasant surprise with his aggressive play. He had 5 sacks last year, I'll go out on a limb and say he doubles that to 9-10 this coming year.
London- I think will have a breakout year and will add another threat to the WR's like Bess did last year. I see him as a situational player maybe 3rd down type of guy or part of spread formations. I see him making some key plays this year.
Wilford- With the dissapointing year he had last year , I think he has something to prove this year, I'm still 50/50 on if this will happen but it could be possible.
J Allen- He will be pushed by E green and the draft picks coming in (I expect at least 2 DB's drafted). With the dogs nipping at his heels He will improve his play or he'll be gone.
Cobb- He showed flashes of what he can do last year, I think you will see more of Cobb in the dolphins game plans this year and he will not disapoint!! With a 3 headed monster RB ( Brown, Williams, Cobb) all 3 should stay fresh through out the year.
I think there will be others from last years roster that will improve as well But the ones I posted above will make a huge impact and will be a pleasant surprise