Repurpose old ones! Sounds obvious, but this has been a mainstay of Team Fortress from the beginning.
I’ve already mentioned that 3 popular (and one rarely-played) TF2 maps were originally maps in Team Fortress Classic, ported over due to their popularity. TF2 itself is quite fond of self-plagiarism, with there being no less than three versions of Gorge (four if you count Itemtest), Badlands, and many many maps with two different versions.
These reskins come for a number of reasons. Sometimes Valve wants to showcase a new game mode and so they release some brand new maps along with a few revamped ones. This is how arena_granary, koth_sawmill and ctf_foundry came to exist, for some examples. This is a good idea because it can help establish the new game mode by giving players a tried-and-true map to compare it to, so they can take all they’ve learned about the old map and see exactly where this new one differs. Other times, Valve releases an event remake in the same game mode, usually for Halloween. This has become something of a tradition, to the point that people expect this to happen every year and have fun trying to predict what remade map we’ll be getting.
And then sometimes Valve just decides what the hell and releases a remade map just to bolster its map count. koth_nucleus and ctf_well were born for no better reason, and honestly this isn’t a bad thing. New maps do nothing but give people more options to have fun in. Sure, its better to get completely brand new maps, but its better than nothing.
Training mode is the one place where Valve seems to have decided that only remakes will come out. tr_dustbowl and tr_target are both derived from older maps, and I think that its for the aforementioned familiarity boost. When you’re making a map designed to teach people how to play a class, you want them as comfortable as possible, and its a relatively safe bet that Dustbowl is among the top two guesses for maps they’ve probably played before.
So do more of that if you like, Valve! New maps are new maps. One of my favorite CTF-dedicated servers has incorporated ctf_gorge and ctf_foundry into their rotation without the Mannpower elements, and its provided a breath of fresh air from Turbine and 2fort. Every gameplay-advancing addition, no matter how small, is a welcome one.