The Rule 5 draft is next month. Jessica Camerato's newsletter notes the roster challenges for the Nats:

Among Washington's top 30 prospects, as ranked by MLB Pipeline, outfielder Jeremy De La Rosa (No. 10), right-hander Jackson Rutledge (No. 12), right-hander Aldo Ramirez (No. 18), left-hander Jose Ferrer (No. 23), left-hander Matt Cronin (No. 25) and catcher Drew Millas (No. 30) are eligible. 

Other notable prospects include right-hander Mason Denaburg (2018 first-round pick) and right-hander Jake Irvin (‘18 fourth-round pick). Shortstop Jordy Barley, right-hander Richard Guasch and southpaw Tim Cate also were not protected when they became eligible last year. 
 
The Nationals' 40-man roster is currently full, with seven players mentioned above set to become free agents. On the 40-man roster, pitchers Joan Adon, Gerardo Carrillo (No. 14 prospect), Francisco Perez, Tommy Romero and Seth Romero; infielder Lucius Fox and outfielder Yasel Antuna (No. 19) finished the season in the Minors, with Antuna and Carrillo pursuing their big league debuts. 

My guess:
- Waived: Adon, Fox, Perez, Tommy Romero, Seth Romero
- Signed: Cronin, De La Rosa, Rutledge, Ferrer, Milas

I assume Antuna has no chance at selection given how poorly he hit both in the regular season and so far at the AFL.