Okay, Lord_Z, I gave your map a little bit of playing time over the past few days and have a few comments. Let me start, though, by saying that the map is pretty impressive and will certainly be a worthy addition to the project. It looks wonderful and blends several different themes effectively. Pretty good with respect to gameplay as well. Not too easy, not too difficult.
That said, there's still room for improvement. Mostly bugs or minor gameplay issues, though. I played this map in PrBoom Plus 220.127.116.11 (I think, might be 18.104.22.168) complevel 9, and I have a feeling there are some bugs in this port that you wouldn't find if you used Zdoom. Since this is a boom compatible project, it's worth going over the maps with prboom plus or similar. Anyway...
1. There were several HOMs throughout the map. Taking screenshots didn't work out, so I'll have to give you the locations. As you come into the secret red skull room the computer consoles in the wall of the corridor had HOMs. There was also one in the room of the secret exit, and one in the secret room with the megaarmour in the vent. I think that was all.
2. That door that you have to hit a switch behind a desk (just past the nifty outdoor road and forcefield area) can end up being permanently closed with no way to get from the side with the BFG to the other side. The switch on the side with the BFG can only be used once, but it makes the door stay open for good. Where's the problem, you might ask? Well, if you then use the switch on the other side even once the door will then close after a while and can never be opened from the BFG side again. This happened to me once purely by accident, and it isn't as difficult to do as you might think, so make the switch on the BFG side the same as the one on the desk and this won't be a problem.
3. Gameplay issue here. I note that you don't get the plasma gun until the very end of the level (and only then if you find the right secrets) whereas the BFG comes much sooner. I think it might be more appopriate to switch the places of these two weapons, since there are no fights big enough to require the BFG, and the plasma gun is the better general purpose weapon.
4. That reminds me. Even though I visited all of the secret areas (I know this because I used DB to find them all), I only got 7/8 secrets. I'm pretty sure that the one that didn't register was the sector you go through as you jump into the room with the plasma gun. Not too sure why this was.
5. The secret exit contains a lava pit which there is no way out of to the best of my knowledge. It's generally a bad idea to make slime pits with no way out. Put a teleporter or a lowering platform there. It's possible there was a way out that I missed, but best to check with you, I think.
6. You can actually pick up the red skull key as you're going up the elevator with the cacodemon in it (when you walk through into the control room with the blue key switch, to be precise). Either move the RSK a little bit so you can't do this, or relocate the secret entirely. I don't like it very much since you would probably never guess it was there without Doom Builder.
7. It isn't clear where you need to use the BSK, since the control switch isn't marked.
8. The revenant and hitscanner hidden behind the walls in the sewer are annoying. I think you should make their presence more obvious.
9. Make it so that the door joining the bottom of the road and forcefield area to the room just outside the starting area will open once you have the yellow key (or maybe once you've used the yellow key to get through the other door, if you want to force the player to go through that one first). It would make backtracking through the level a lot more convenient and fun.
10. This is a pretty subjective matter, but I don't like trying to open doors in the vent where there are only a bunch of door textures instead of knowing which are the real doors. Might be better if you just got rid of them or at least did something to show that they're broken and won't work.
That seems to be everything for now. If I think of anything else, I'll be sure to let you know. I liked the secret exit, by the way :) .