Talknats notes that we've got a lot of rule-5 eligibles next year:
Rule 5-Eligible Players from MLB’s Top 30 Nationals Prospects: Jackson Rutledge (#6), Matt Cronin (#15), Jeremy De La Rosa (#16), Drew Millas (#22), Jackson Cluff (#24), Israel Pineda (#26), Jordy Barley (#28), Mason Denaburg (#29), Jake Irvin (#30)
Cluff, Barley, and Irvin seem to be the most likely to remain unprotected, meaning that we need to find 6 slots on the 40 man for guys likely to be in the minors next year. I imagine they could also leave guys like Denaburg and Rutledge unprotected as they aren't going to be on major league rosters next year either.
TN predicts we free up space by DFA'ng Antuna, Barrera, Seth Romero, and Donovan Casey (I think Casey gets more time).