Don't the Nats have to pretty much dump next year's UFA's now if they can? They could gain some confidence in a series with Miami, but I fear it'd be false. Harper, Murphy, Gio, Madsen, Weiters and even re-flipping Herrera. Teams know they are rentals, but it seems there'd be some value there. Even if the Nats intended to make a qualifying offer to any of them, my understanding is that the draft compensation is not as valuable due to the team's recent luxury tax status.
With Scherzer, Strasburg, Doolittle, Rendon, Turner, Soto, Eaton and Robles and whomever they may acquire in FA in 2019, the "window" is necessarily slamming shut after this season. It seems to me that even if the above list only netted the team one or two potentially valuable pieces and some filler, it'd be better than nothing. Maybe they keep the valuable new prospects or maybe they're flipped at next year's deadline if the team is in it.