I just noticed on subway that there is pretty much just one solid line. On the map above, there is one line that splits into three. Flat arena also consist of a single line (for the most part).

For me, personally, this gives a lot of limits. This means there aren't really any "flanks" or ways people can move as a group without just getting picked off along the way. It makes it so that you hope you're always going on a side where the enemy isn't defending. In some cases that can just be luck.

There's two you might want to go with.

The first method is just as a simple map. Make a design to a map rather than "flat arena". I can't argue with "most popular" but look at something such as subway. Imagine if there were two subways. Now add in the fully furnished stations on either side who each have the flag at the back. Rather than just one line connecting the subways, add multiple pathways that wrap around. Avoid having too long of a straight line, and if you feel like you do, add a room to break it up. Add lots of detail. Detail is GREAT to hide against, waiting for enemies to come attack you or waiting for the defenders to leave, there will always be somebody to sneak up on if you're patient. This means things to hide behind, under, etc wether it be under the subway rails, next to a vending machine, etc. Lastly, consider layers. If there's too many people coming through the main route, is there an overground or underground route to cross over as well? You want to make it seem like the user has a bit of a choice when leaving spawn on what he/she should do rather than just run for the enemy flag.

Choice two is similar, but it's a bit more open. Rather than just rooms, make hills to guide the pathways. Add trees, decoration, rivers, etc. Sure, the predefined, relatively flat pathways will still be used, but it might be worth your time to cross from one to another over a hill. This makes the entire map open to opportunity rather than some fixed rooms.

Again, they don't have to be complex, you just basically want to avoid a single straight line (even if it splits into three). Make a map where certain parts of lines connect to a different part of them, or where the spawn is a little offset to the point where most people won't go through the flag unless they're trying to defend it.

This is all my opinion, and perhaps that's not what your server is looking for, but I felt I'd drop the advice :)