I don't really feel there's a need for the Gauss Hog to be added personally. The Rocket Hog already fulfills any point there'd be towards implementing it.

As far as suggestions go however, I admit I was expecting to hear Foehammer's missing dialogue audio from the opening of 343 Guilty Spark. Yeah that's not "essential" but given the magnitude of everything else added, I was just kind of expecting it.

There's also some AI behavior that eats at me. For one, the Marines aren't programmed to fire at the now-destructable Spirit Turret, which is a real problem when you're driving a Warthog or a Spectre and require the Marines for support. They ARE however programmed to fire at the Engineers, which is a real problem for when you find them in Assault on the Control Room because there's not a lot of room to avoid their explosions and you're better off NOT destroying them. It might just be better if they didn't target the Engineers. Another thing, and I don't know how this could be attributed to the mod but it never once happened to me in the original game, but more than once when I find the Scorpion in AOTCR and jump out of the Warthog to get to it, I end up getting killed because the Marine I had in the Warthog's gunner seat is shooting at me because I just happen to be inbetween them and another enemy.

I'd call this an optional suggestion, but I feel there could be some mileage in implementing one of the original ideas for the crazed Marine in 343 Guilty Spark, about how he would commit suicide with his Magnum after you've put some significant distance between him and yourself after entering the room. I realize that can be a touchy subject though, so maybe instead just have something like if you revisit the room after the Flood have gotten out, there are signs the Marine was dragged away from his spot. Again, more an optional suggestion than anything.

The added scene in Two Betrayals where you can teleport back to the Control Room to save the remaining Marines from the Flood is awesome, but it's not exactly clear on what you have to do or what's going on. I understand there probably aren't any sufficient audio clips to have the NPCs explain what's going on or what you have to do, but it could at the very least use a text prompt similar to how The Library explains what you have to do in specific situations. Even now I'm still unclear as to wether or not the Flood spawn after the Marines evacuate is infinite or not.

The secret Scorpion in Two Betrayals, while neat, is a bit hampered by the difficulty in getting it over the barrier just before the last big stand against the Covenant when you have to hijack a Banshee. Getting the Scorpion over the barrier without modifying the level geometry is such a struggle, and just straight up not taking the Scorpion into that last segment leaves having it at all feeling frustratingly brief and hollow.

I do love that a lot of previously inaccessible areas in the game can be accessed now, either for genuine reward or just flavor, but I find it's a little too easy to get lost in The Maw now. Often times I end up completely confused as to where to go because too much looks the same and there's no clear indicators of paths taken or not taken. Just a bit more setpieces to determine the direction you SHOULD be going in could help to mitigate that.

I've posted some of these in the comments, but I'll reiterate here just for posterity.

The only problem I've noticed with the sentinel beam is that the beam itself hits waist-high cover in front of you, so it could probably use a bit of tweaking. In Halo 3, I think they made it so the beam passes through the very tops of objects in front of you.

I'm of the opinion that the sniper should be effective against flood combat forms even if you don't hit them in their core. It's not a super common weapon anyway, and having to line up head-shots on an enemy type that's meant to overwhelm you with numbers feels a bit counter-intuitive to me.

I haven't checked other difficulties (or vanilla) in a while, but on T&R on legendary, the shuttle bay door button on the second floor doesn't seem to serve a purpose. I think it would be nice if you could use it to get some reinforcements if you're running low after the big hangar fight. I've gotten to the point where I can usually make it through the level without needing a refill, but it would be nice as a backup in case you have a bad run. Plus, some of the subsequent encounters seem to be designed for having at least a few allies with you to provide support. They can be kind of tedious if you're by yourself.

I have a bit of a problem with how the banshees show up on a couple of levels, mostly AotCR. In some of the encounters, the banshees will swoop in from above, and the very first thing they do is launch a banshee bomb. If you're in a fast-moving vehicle, like a warthog or a ghost, it's not too hard to avoid. But if you're in a tank (like most people probably are), chances are you're gonna lose some marines (and a few pips of health) to a threat you didn't even know existed until it's too late. I've found that even if you have a pretty good idea where they're gonna come from, it can still be difficult to intercept them due to the fog and snow. Easiest solution I can think of is making it so their opening attack is their plasma cannons instead of a bomb, that way people can still get shot without incurring big losses. If you don't take them out quick enough, then they start bombing.

The shadow and wraith are cool, but they pale in comparison to their human counterparts, mainly because of the vehicle health. They both get absolutely shredded by shade turrets. It's a shame, because gliding around in a shadow with an extra passenger feels pretty badass... until you encounter another vehicle or turret. I'm not really sure what you could do about that, aside from altering their health when controlled by a player, and I don't know if that's possible.
I do at least think the marines could use a buff to their firing patterns when in the shadow turret, since they often miss their shots. I think it would also be useful if the wraith had halo 2-style plasma guns on the secondary fire.

Some of the marines on the lower levels of the ziggurat on Two Betrayals don't seem to contribute much to the fight, they just kind of hide behind cover the whole time. Not sure if it's a problem with pathing, sight lines, or attack ranges.

I'll come back and put more if I think of anything.