K, I’ll mash together some replies here, but if I miss something, I’ll follow up with more.
A model of the moon would be easy to make, but challenging to set up in the map because there are quite a few windows throughout it. It would require lots of space, and it would look odd if you can see the model from some windows and not others. Asteroid models would be cool, but might look odd if there isn't a matching moon model. Also, the additional epoly would have to be taken into consideration. I'll probably release the first version without models, and think about that a bit more.
The map has a fair amount of vents and hiding spots, but I didn't want to make it too complex and hard to learn. Therefore, I placed them where I thought it would make sense strategically (ie a vent exit located where a lerk or gorge can use it to defend an important location, or where a skulk can use it to flank) but held back a bit on how many I added. However, I can always revise it based on player feedback later on.
Also, balance must be taken into consideration when adding vents and shortcuts. The following image will help illustrate my thought process with regards to map layout:

At the core of the map, there are two resource nodes. To make it balanced, I wanted the path from each hive and marine start to those nodes to be roughly the same distance. It's not exact, of course, but within reason to prevent one team from starting with an obvious advantage.
At the same time, I want to make sure that the distance between each of the hives and marine start is roughly the same, yet longer than the distance to double. As you can see, the yellow path from Marine Start to Cargo is longer than the other paths, but I think that stopping off to cap the two nodes along the way compensates for that.
Finally, I want to make sure that the remaining resource towers are evenly distributed throughout the map. As you can see, the hives and marine start are on the straight sides of an octagon while the remaining resource towers are roughly in the middle of the angled sides between the hives and marine start. They are also in enclosed areas so that they cannot be destroyed from a distance. Hopefully, this will prevent one team from getting an unfair resource advantage.
If I add more vents simply to give more options without taking the above into consideration, it would be to the detriment of balance. I think the octagon shape of the map lends itself well to these restrictions.
I've taken some additional shots of waste halls (2nd picture in the original post) to expand the view and explain it a little better:
In this first picture, you can see the resource tower in Waste and, through the doorway, a bit of Waste hive:

The vent in the previous picture leads to an opening under the catwalk that is on the right-hand side of the original pic, which I circled in red. The vent continues around the corner to the hall on the left side, and aliens can jump up from the sides of the catwalks:

In this next picture, you can see a vent opening, under a pipe, inside waste hive that is circled in red:

That leads to the area in this next picture, which is above the hall on the left-hand side in the picture from the original post, and the grate is a func_illusion so that lerks can spore/umbra through it and use it as a defensive position:

The other vent on the right-hand side of the grate in the previous picture leads to this exit, and aliens can thus pass through that defensive position and carry on to another location:

Above the pipes, there is a catwalk that jetpacks can use to sneak up a pg, or aliens can use as a defensive position:

This next picture shows where I'm considering adding a vent that would also allow aliens to get above the grate on the right-hand side of the picture in the original post. I hesitated because it would just lead to a dead end, but now I'm thinking of adding a vent from there that will lead to the catwalk above the pipes, and thus have better flow while adding another defensive position:

These final two shots give a better view of what each end looks like. In the first picture, you can see that the walls actually curve out a bit. It's a good point that using one texture over a long hallway without details to break it up should be avoided, but as you can see in both pics, the area on each end is small, which wasn't really visible from the original pic:

Making some of the lights so that they're turned off is a good idea, and I do that in other areas of the map, but I kinda like the brightness in waste halls because it's deceptive. To a marine, a bright area gives more of a sense of security, yet aliens can attack from just about every angle of those halls.
To be continued....