If the Isles make one move ahead of the deadline, I hope it is for a solid depth defenseman.
Ideally a LHD (or a RHD adept at playing his offhand), who can be paired with Dobson for the rest of the season and help mentor him like Chara and Greene did previously.
At this point the "best" move might be an inexpensive addition who can help unlock Dobson and prod him along to reach his potential.
And if the Isles can land a defenseman who can play the left or right side, it provides insurance in case Mayfield prices himself off the team, and maybe that player is paired with Romanov and helps bring him along next season as well.
Mayfield brings a lot of things to the ice, however, teaching and guiding younger players doesn't seem to be a strength. I haven't really seen him elevate anyone's game, so a capable leader on defense could be the best move and very affordable (say for a second round pick).
If you can squeeze a couple years out of that player, like Greene before the wheels fell off, it would be a big boost to the short-term and long-term for the Isles.