Rizzo isn't going to return with the same team. There has to be a major move, like every offseason
I'm thinking the OF will go without an addition unless the Nats package Taylor in a deal but if he has turned a corner and is a .260/.320/.480 with exceptional Fielding in CF, then he is a cheap piece you hang onto with Eaton. Harper, Taylor, Eaton from left to right is a good defensive and offensive OF.
IF is still set with Rendon, Turner, Murphy, and Zim. Catcher is the obvious upgrade but they would likely have to move Wieters and part of his salary which likely won't happen.
Scherzer, Stras, Gio, and Roark are locks. Roark doesn't make a ton and Gio probably has enough value to stay because a hole needs to be filled with the fifth guy. I don't see the Nats spending big money for Darvish. Arrietta might fit with the Boras connection but he looks to slot behind Stras but get virtually the same money. Otani probably can't come here because of the International spending limits. I could see a trade for a mid rotation guy like Samardzija or someone similar.
Bullpen is an obvious spot for upgrades. Davis, Holland, and Neshak head up the list. I think Doolittle sticks as closer so any of those guys as 7th or 8th inning to go with Madson is solid.
Bench could use help, but I guess Severino replaces Lobaton as back up catcher. Difo replaces Drew. Bringing back Lind could definitely work. Not sure how much Kendrick has left in the tank or if he wants to be a bench guy.
The blockbuster could always be a Harper extension, but I think that is delusional.