This is a tricky question, as there are three main methods the Nationals can use. They can (and almost certainly will) honor players who played with the team since their move. They can honor players from their days as the Expos (and, as we will see, they aren’t out of these candidates yet). They can also honor players from the last two Senators teams, which they’ve already started to do in some ways. I think some combination is the most likely.
Let’s start with the former Expos. For the hitters, there are really only two good choices left. The more obvious one is still playing. Vladimir Guerrero came up through Montreal’s system, reached the majors in 1996 and played with the Expos through 2003.
Guerrero has 36 fWAR with them, placing him fifth all-time, and 32.4 bWAR, placing him fourth. I would say it is very likely that Vlad will see his number retired (if not with the Nationals, then with the Angels, or possibly in Montreal by itself, since the Expos retired numbers are apparently displayed at the Canadiens’ arena).
Should the Nationals do choose to recognize No. 27, I have no idea what their time table would be (would they do it immediately after he retires, or wait for him to reach the Hall of Fame, or something else entirely). If they are going to continue recognizing Expos though, I would say he is the only lock.*
*Although, seeing as he shares his number with current National Jordan Zimmermann, they might have to wait a bit to retire his number. That may hurt his case. On the plus side, they can be the first team with two numbers retired twice.
Tim Wallach is the only other player with over 30 of either WAR. It may say something about my lack of knowledge on Montreal’s team, but I had never heard of him until I wrote this article. Apparently, he is a third baseman who played with the Expos from 1980 until 1992, and leads the franchise in plate appearances, hits, and total bases, among other things.
Wallach has 31.3 bWAR with the Expos, fifth in franchise history, and 38 fWAR with the Expos, fourth in franchise history. He obviously doesn’t have the star power that Guerrero does, so I doubt that his case will carry over as well to Washington. Plus, he’s already been out of the game since 1996, and there obviously wasn’t enough of a push to get Montreal to retire his number when they were still there. A decade and a half out of the game, with your biggest ties to a city that doesn’t have a team anymore, is not a good recipe for a retired number.