There is already a thread like this, and I think it should be stickied, honestly. Where is it...it's called Forge Ideas or something. Look it up. Anyways, try to design a map around a fortress and a team attacking it, designed for one bomb or one flag. Those are particularly easy to make.
A giant duck that breathes fire (lolwut?). You have it as infection, while the humans drop spawn from the air, land on top of the mancannon, and are shot across a gap in front of a giant duck's mouth. Then you have an infected alpha zombie spawn with a flamethrower and tries to burn the humans as they fly by. If he missed a human, than they fall to their death and respawn back above the mancannon to be launched over again. All of the humans who become incinerated respawn as a zombie in a spectator box to watch the whole thing from a high altitude. You could do it in the sky bubble on Sandbox. Yes, I just thought of that on the spot, but hey, it's original to say the least.