Duvall's defensive versatility gives contenders more options
We have talked a lot about Duvall's bat and rightly so as that is going to be what drives the demand for his services the most. However, the fact that he can play all three outfield positions pretty well is an added bonus that could entice more teams to pull the trigger especially analytics-forward teams that value defense highly.
We will use the Brewers as an example here. Milwaukee's offense is truly bad with a team wRC+ as of today of 89. However, the only reason other than dumb luck that they are in contention at all is because they have been the best defensive team ($) in the league this season. Adding Duvall would, in theory, boost the offense while also not giving up much if anything on the defensive end.
A team like the Mariners who could use some outfield help and who could also use a defensive upgrade in the outfield is likely to value Duvall highly. The Astros are famously analytics driven and could see Duvall as exactly the kind of short-term move that would help them on both sides of the ball.
The trade deadline is mostly about patching holes on the roster for the rest of the season. Teams in need of centerfielder could play Duvall out there even if it isn't his ideal position. Need a guy for either corner spot? Duvall can do that, too. This isn't a guy that you have to play in LF because all he can do is hit. Duvall's defensive versatility is a huge selling point that will appeal to a much wider range of teams.