After I read all these replies, I've gotten some nice idea's
I guess the most know what ATB is. Assault The Base. And I don't mean Assault-levels, but CTF levels. It should be 2 levels: one with the blue team flag for the grab and the other for the red team flag. The idea is simple:
Team Red Assaults the Base
Team Red will start with a trigger ON, wich will give them entrance to their own base.
Team Blue won't start with a trigger, so they only can protect their own base (and the paths to it).
While Team Blue defends their base, inclusive all the roads, paths, sewers , doors etc etc who lead people into their base, while Team Red tries to get in. Of course Blue gots some better weapons, and Red has to do it with less. As soon Red is at the BlueBase, he/she (or it) sneaks in and hopes to find the Flag, without being noticed. As soon he got it, he should run for his life, with loads of the Blue Team behind him. As soon he enters his base, Blue should know they lost. 2/4 points should make the teams change. The server should use that Controller or what it was to change the lvl if the game is roling to long.

Team Blue Assaults The Base
Now the roles have been changed, Team Blue will notice how hard it can be to get into that base Of course there are enough sneaking routes (you have even heard me saying "sewers", right?). Of course Red Team gets some better weapons and again, it's time to assault these cowards. Now Blue gots a trigger ON, wich will be letting only Blue someones in. Etc Etc, just like the text of when Red has to attack.

Shouldn't this be fun? I think so. Maybe I'll just make a betaLVL from this and see how it will turn out

And BTW, the idea's from FQuist(those construction and destruction), Mikeejimbo and the one from BlurredD sounds fun too.
And BoggyB, I think you have given me a new idea I think. If we make a SMALL level like the one from Kirby's Dream Fountain, and everybody gots TNT. You can't die, untill you will be blown from the lvl. You will hit a warp, wich will send you to a room, and you'll have to wait there untill everybody is out (or there has to be one left). It's like the Ground Force lvls, that you will be in a square sucker tube, and the lvl will go further untill the server gets that diamont and goes into the exit. Maybe this is a fun idea, for creating the BEST SSBM levels who are small to play with this :P :P