I've seen a map with a similar idea to this doing the rounds, I think it's called 'CallOfDuty'. It's a seige and quite small, and in my opinion not very good.
The idea has crossed my mind before but I think there are a great many drawbacks to it, even though some of my favourite levels on other games have been based on a similar concept. Those which immediately spring to mind include:
- Vising would be a NIGHTMARE, so r_speeds would be huge.
- Given the spray and pray nature of DP, trench warfare doesn't exactly lend itself to DP's gameplay strengths. I.e it would be a bit slow and the weapons' natural advantages/disadvantages don't draw any parralels to the sort of gameplay dynamics you get, say, on Call of Duty 2's El-Alamein map.
- If you are attacking the fort to grab the flag, and presumably have to escape back to the boats with it, it might be hard to get balance issues right for attacking/defending teams. I mean, consider the REAL d-day's k/d ratios... 43,000:23,000 allied to axis KIA's. Not many people will want to play a map where they die twice as often attacking as defending! (probably...)
- What with our limited palette of textures and such, it might be hard to make the routes memorable enough so that people aren't hopelessly lost in a maze of tunnels for a long time before they get used to its complex layout.
Good luck if you undertake it anyway, I'd quite like to see a proof of concept.
- Dagless M.D