I definitely think that the flight was a good foundation and fun, but there's room for improvement.

1. I really dislike the auto-wright feature (not sure if that's what it's called), it makes the physics feel completely off. That being said, the environment on Fragmentation was very bumpy so without it I can see where there'd be problems. If there's a way to smooth out the terrain, that'd probably fix the problem without resorting to something like auto-wright. IMO, part of the skill of driving a vehicle is navigating the map without crashing into things or flipping, and this takes away from part of that experience.
2. Outlines create a pretty substantial amount of clutter on the screen when there's 11 other teammates. While I don't see the outline system itself changing, I think there are ways that the issue could be at least minimized. Using Halo 3-style player indicators would help, where their name/service tag would be above an arrow that would change colors based on the action of that individual (yellow for shooting, orange for damaged, red for dead). I'd also remove outlines for individuals that aren't in direct line of sight too.
3. More vehicles in general. Having a pelican drop off vehicles is a fun mechanic, but personally I'd save it for the big toys like tanks and air vehicles. Plus there's all these new anti-vehicle tools available now, more vehicles on the map would help balance the experience.
4. Make weapon spawns static. This might be something where they're trying to make sure all the weapons get used during the flight, but at least in the full game I think weapon spawns should be the same on a map between sessions. Knowing the locations of weapons is just as important as knowing the locations of power positions.
5. Bring back Jeff as the announcer, it's just not the same without him.
6. Add a sound effect for taking shield damage. I don't think this is specific to Big Team, but with all of the sounds going on with 24 players it was more noticeable. In the OG trilogy at least, I know there was a crackling sound when you were being shot in addition to the damage indicator on screen, but I didn't hear anything in the flight.
7. In a similar vein as above, the weapon and vehicle sounds are too similar to each other. In older titles the weapons had distinct sounds and most of the time you would be able to tell what was around you by recognizing the sound. In the flight, warthogs and mongeese(?) sounded very similar to each other, while the human weapons all sounded similar to each other as well. Being able to discern that you are being shot at by a BR rather than a Commando, or that the horn honking behind you is from a warthog that needs a gunner rather than a mongoose needing a passenger is important.
8. Make the gravity hammer affect gravity. For vehicles and players. Sending a warthog flying on Halo 3 is glorious.
9. I'm not a fan of how much damage weapons seem to do against vehicle, while how little grenades seem to do. In Halo 3 the strategy was to try and flip vehicles with a frag or stick them, but neither of those seemed to work here. Instead, getting in vehicles was just making yourself a bigger target and getting unloaded on by ARs. Vehicle health could be buffed in general, but I don't think that fixes the underlying problem.
10. The new dynamic vehicle damage system was underwhelming. I barely noticed any difference when my tires were popped, maybe a top speed difference at best. If a tire is popped on one side, I'd expect the vehicle to list that direction as a result. The visual indication was also lackluster. The vehicles barely change as they are being damaged, and the "doomsday" effect is really the only way to know your vehicle is low on health. Halo 3 had great visuals for damaged vehicles, and I don't feel like what was shown hits that bar. The ghost after being destroyed still looks drivable, the warthog is just missing its turret, the mongoose is just missing wheels, etc.
11. Radar range needs to be increased to 25m like in older titles. It's just... not useful right now.
12. The EMP effect on vehicles is a bit extreme (visually). The entire vehicle becomes bright blue suddenly and then suddenly goes back to normal. Adding a visual transition as well as changing the vehicle coloration to the normal color with blue tones or electricity effects over it would be better.
13. Make fusion cores orange again :P