There isn’t much of a strict “design ethic” in BMS outside of a few basic principles. It doesn’t have a complex “meta” like CS does, or anything like that. You run around, get good weapons, and shoot stuff. Though I hate to always be plugging myself - I would suggest checking out my Gasworks blog - it provides some useful insights into beginner mistakes for mapping for BMS specifically.
It’s quite basic, at least from a gameplay perspective. Avoid dead ends with very few exceptions. Incorporate verticality in interesting ways, don’t just have it there as a token design thing. Our game is significantly more fun when verticality is involved. Make sure every space serves a purpose and adds to the map; ruthlessly cut anywhere which doesn’t. Make sure there are no ultra strong spaces on the map which see too much play, or if there are and you like that - incorporate them into the design in interesting and unique ways (like the RPG fort on Gasworks, is a good example). Don’t give the player too many options in terms of routes/room entrances/spaces, etc (this is something a lot of custom maps are guilty of) - it creates overtly chaotic and un-fun gameplay. Don’t limit the player too much - it makes the map boring and unvaried. You have to strike a balance, something I think our stock maps generally do very well.
Then you have to consider weapon placement and map flow. Players are drawn to the powerful weapons (Tau, Crossbow, Gluon). Don’t put them (generally) in spots where they are really powerful. Try and put them in places players may otherwise not wish to venture, but creates interesting encounters and mechanics when they do. A really good example of this are the Gluons on Undertow/Bounce, in my opinion. It should never be too hard to acquire a weapon (otherwise it becomes a Glock and Nade spam fest), but it also should never be too easy. Our maps tend towards it being a little too easy, but that’s because we’d rather things were a bit too chaotic than too boring - though obviously a perfect balance is preferable. Also be careful about how players can move between powerups. In some of our early tests for MP, players were finding some really strong routes they’d just run laps around to become super godly. We had to eliminate those.
Players should be encouraged to move around the map a lot, but take stops while doing so. Too much movement, and you get a chaotic and jumbled map where players don’t really have an aim or know where they’re going. Too much camping, and the map is boring.
Basically, just keep the map well balanced. And if you do take it out of balance, think of ways you can do so which keep it interesting.