I think the weakest map in the rotation this season is sadly gravelpit. Gravelpit has been a staple in the map rotation set since Season 1 and has always proven to be a great tie-breaker map due to the stopwatch config. However I just feel that there is a bit too much predictability in the strategy and timings now. Sure every map has that predictability but it is especially so nowadays on Gpit. Also there isn't that dynamic push-pull and a constant tug of war like you would see in a 5cp or KOTH map. Still, there were some great Gpit matches this season and being the map that it is it brings some of the most exciting matches from a spectator perspective including FT vs. Vector. That could also be a reason to get rid of the map, as it could be argued that Vector wouldn't have won that matchup on a 5cp. It's also interesting to note that Gpit has been voted out of the rotation in both ETF2L and OzFortress.
Metal works is a great map as we saw this season and should stay in the rotation. It is honestly one of the most unique maps due to the interesting flank from 2 to 3 and the design of the second point is very interesting. Also last point is decent to push out of right now.
As far as replacements maps go, options are limited. To remind everyone, the maps Well, Freight, Warmfront, and Yukon were all voted out of the rotation long ago because last points were impossible to push out of and were stalematey/not fun to play. Well has a second point that wasn't much fun and Freight has spawn issues. Yukon had an interesting mid and is similar to Gullywash because the tunnel/flank cuts behind the main choke. But Yukon's last was extremely difficult to push into as the large geometry meant your uber would get forced early and then easily kited. Pushing out of last was a nightmare as there was only one true entrance from which to push out.
I think one of the best options is to keep the current rotation until a better map is developed. This was honestly one of the best seasons as far as map rotations go.