About the pool, for a while now I've been thinking of a small idea.

See the height where Dusty's feet touch? Across that entire height, you could put a transparent floor (or maybe a hair higher) so while the pool no longer has Doom water, it gives off the illusion/feel of the characters swimming while still showing it's deep end. Therefore it still looks and fells like a pool.
It would also be non-solid to projectiles.
Here's a mockup of a proposed hotel expansion I came up with long, long ago:

Note the bathrooms re-added in the MM3 hall.
As well as a closeup of 2F:

You could also do the following:
*Decorate the exterior of the building a bit more, especially the entrance.
*Add scenery into the halls like lamps ceiling, lights, couches, greenery, and small tables. As for the entrance where Dr. Light works, it could possibly also be decorated with the aforementioned lights/lamps/greenery and maybe a vending machine or two (you can also stick some vending machines outside).
*Color the edges of the walls in the hallways with a different color (more aesthetic and thus not exactly necessary)
*Add my hallway signs (that I also posted in Wily Inc) to mark the hallways of the different games (also aesthetic).

*The rooms of as-of-yet unreleased characters would use custom tilesets and be as professionally made as other hotel rooms.
*Work in more opportunities for health pickups aside from the very slow process of healing yourself on beds, the seats outside, and the showers. Additionally, being in the hot tub could also restore health.
*Use the space north of the pool shown above to implement in something extra, it could be outdoors or indoors (Personally I would suggest converting it into a gym of sorts).
*Add in additional rooms scattered through the hotel, such as a library, a casino, a theater (What's there near the MM4 halls is more of a bar), and a medical center (that's stocked with recovery items). You could also remove the "NOPE" sign and the arrow next to it to conserve a bit of file space.
*The garage that normally could not be reached before if official versions should stock teleporters that lead to different parts of the map.
*I also approve of decorating the parking lot into some sort of actual battleground through the usage of vehicles.
That's what I could come up with mostly for now, even if you scrap this map entirely and remake the map from scratch. (In that case, you could incorporate maybe a secondary entrance or two that players can leave the hotel through (which I added in through the areas by the bathrooms in the old map)
I can help with any spriting/texturing if you like, most of the stuff I posted would be easy enough of rme to sprite assets of.