What players at 21 could impact this franchise right away?

[*ochocincos*]

Most immediate impact will be OL. OL has 1-3 spots where an upgrade is needed immediately. Whichever player(s) from the draft fill(s) the spot(s), the snap count percentage should be 90+%. C is probably the best because RG has Westerman/Redmond/Hopkins competing for that spot still and RT has Fisher and Hart.

Argument could possibly be made for NT but that's still going to be less than 50% of total snaps unless that NT can also be very good rushing the passer to justify staying in on passing downs (unlikely).
A new shiny safety, LB, or DE won't get nearly that high percentage of snaps barring injuries to their position groups.