I'd think Vaughn, Kopech and Montgomery are up there for the Nats. Maybe Vera too. Trading Vaughn would hurt, but it would help clear out our 1B/DH logjam. You could then extend Abreu for another year or two and see if Burger can learn 1B. You'd be decent to solid at 1B and amazing in RF, which is a better combo than they have now.
Kopech I'd balk at potentially. I think Cease and Kopech is our dynamite 1/2 in the future. I wonder if they would take Crochet? They could rehab him and put him in the minors to stretch out as a starter. I think that'd be better for his career.
Montgomery would REALLY hurt - no guarantee he'll keep this up and be great, but I'd be worried about trading away a special player with tons of control for a special player who is only here for 2 years realistically.