Make a map called 'lake of pain'. Have a fishermans shack on a very tiny island in the center of a big lake. Viscount will spawn inside the shack for some better protection. Put a ballista on top of shack with a ladder to get up. Put a palm tree with a homemade treehouse on it. Bots will spawn on the shore from multiple directions and walk underwater.
I see a few major problems with this.
1. Shack has to be large enough to hold at minimum 31 ppl (full server)
2. Shack sounds way too defensible with only one entrance
3. I've tried the whole treehouse thing..... An actual house in the tree.... it really doesn't work that well.
4. Bots walking underwater can drown, or at least they will in a patch or two. It's being looked into.
5. Based on your description, the image that comes to mind of the map is something that I'm very certain would not be accepted for official server rotation. So with other maps in the works, it isn't really that probable that I would do it.
Now, does that mean in all this BS I've just typed, I didn't have an idea to modify your suggestion immensely into something similar to what you want (I hope), but possibly acceptable to the public at large (i.e. the devs and admins and extreme DTV enthusiasts)? Nope. Not at all.
I'll see what I can do.
*EDIT*
I've finished the first map (I think) based on Elio's suggestion about bot flow. I'll post better screens tomorrow. For now, I just wanted to let the rabbit out of the hat so to speak. And no, I did not plan for the map to look like a rabbit.....which honestly kinda scares me.