Oh boy. Basically I'd want a game that could continue the 2v2 style of Halo CE while also being as conducive to BTB and 4v4 as later titles.
A larger sandbox along the lines of H5 would probably be required - the CE Magnum (the real one, not the pretender we have in H5 currently although I do think it's the best 2v2 starting weapon we have despite its inaccuracies) is great for 2v2 but it definitely doesn't hold up so well at the 8v8 scale so you'd need to adjust the starting weapon depending on game size.
At one point I had a spreadsheet going detailing how I'd design such a sandbox but I don't think I ever finished it and looking back I know I was over-complicating it to a degree. Maybe I'll try that exercise again some time and share the results for feedback.
Probably the biggest thing I'd do to any Halo from 2010 onwards is to buff vehicle health. I feel that Reach got the glass cannon Banshee and the tanks (Wraith, Scorpion, Revenant) right but the Warthog and Ghost have been driven into near irrelevance by the system. The Ghost got a welcome buff in Halo 5 thanks to the increased coverage it offers the pilot and a physics system that makes splatters reliable, unfortunately the Warthog manages to expose its gunner more than ever and the bevy of hitscan weapons make removing him before he can do anything extremely easy - I'm not even sure how much punishment from small arms the H5 hog can take because that's probably the only thing actually less efficient than using the Warthog itself.
Ideally I'd want to land on some health level which preserves the Reach system (health regenerates as far as the maximum for the current visible state of the vehicle with the occupant's shields) whilst not making small arms the most reliable way to shut vehicles down.
Or, if I had a time machine I'd go back and rewrite history to make Halo 2 be a continuation of the principles in Halo 1 rather than a bastardisation of them. I strongly believe that Halo would have been so much better off had the BR never come into existence and the Pistol never nerfed into oblivion (13sk from a 12 round magazine, really, Bungie? Overcompensating much?).
I totally agree with all your points about the vehicles being way over nerfed in H5 especially the Warthog as you said which should be the staple vehicle of any Halo and should be feared or atleast respected, but no matter how good the driver or gunner is the hitscan 99% of the time gets the gunner killed within seconds except on the fantastic map Scavenger which is the only map that gives the classic chainhog a glimmer of hope for a descent hog run.Also I cant believe how useless the Wraith and Scorpions machine gun turrets have become, was it really necessary to put a rediculous amount of bloom AND overheating on them? I always thought since the Wraith turret has always been so woefully inaccurate and the bullets so slow that they should give it atleast the advantage of never overheating. Also I think its an absolute joke that you can no longer shoot an enemy hijacker off of you by shooting a nearby wall, killing them with the shrapnel of the explosion. I mean that was such an unessacary change because by doing so they can just have it severly damage the wraith or scorpion from the explosion but atleast with quick thinking you could stop the first highhacker who boards the front of your vehicle.. Also how epic was it in Reach when while in a Wraith you hit the thrusters and a Warthog/ghost or mongoose who was charging your front gets completely obliterated by the sheer size of the Wraith and collision of the impact? It's safe to say that the Wraith in Reach was by far the best and the H5 version looks and plays pathetically weak in comparison. Lets not get me started on the Mantis.. I don't think I have sucessfully stomped a player or vehicle yet in the darn thing even though I seem to step directly on them at times.. To make matters worse for the once mighty and powerful Mantis now you can be easily boarded at any angle which surely is screaming for a patch; at the very least they could make it where emps dont disable it for quite as long since its laughably easy to emp it over and over again from across the map until you are close enough to highjack it.. Throw in the terrible driving physics if the mongoose that flips too easy (compare to H3 which had the best mongoose) add in the fact that we no longer have a UNSC flying vehicle which we could be making epic Flag or Bomb runs in or getting amazing kills in,and you start to notice just how severely lacking Halo vehicles and functionality has become in H5 compared to its last few Predecessors. I think Reach had vehicle balancing just about right minus the crazy flipping banshee that was a bit OP.I do welcome all the vehicle variety in Warzone but thats not core Halo and 343 has made some welcome changes and added features to vehicles such as being able to switch seats, and the awesome gungoose which supplies your passenger with infinite grenades is a blast to ride.But im not going to go into that because this is about what I think should change.One other totally sensless issue to have is the fact that Wheelman no longer count as assists which to my knowledge ever since Halo 3 maybe H2 they counted as an assist and rightfully so since you are driving your gunner around assisting and ifinding him/her targets and helping to manuver them around the battlefield and the enemy so they dont get killed standing still(Really hoping they update this in the Hogwild update; if not by then then I guess it just isnt happening. .) I think since hitscan isnt going to go away that they should buff the gunner just a bit in the Warthog, Im not sure how else to balance it out and make the standard hog a viable weapon again, but a small armor buff or perhaps slightly faster shield recharging while gunning in the hog could help. Another cool change that I just thought of is for the scout hog; why not have it where your passenger gets infinite ammo with just the basic weapons, the AR,Pistol and BR? That would make it a viable more versatile vehicle which currently now is mostly used just for quick transport or splattering.
NOW heres a big Halo dream of mine but one that I fear that will never materialIze in the Halo games: I think it would be so awesomely Epic to have an objective on a Scarab where one team preferably Elites if we EVER get them playable again would have the ability to drive the scarab around a large map while the other team outfitted with UNSC heavy vehicles would attack the Scarab and either have to plant a bomb on it or capture a flag from it.Ofcourse just like in H3 you could bring the Scarab down to its "knees" by firing at weak points on its legs; thus making it easier to hop aboard and plant the bomb.Ofcourse teams would take turns attacking and defending for balance and the Scarab would blow up in Spectacular fashion as it did in Halo 3 , also killing everyone who was aboard also crediting those kills to the bomb planter to further promote and encourage people to play the objective, which as many of us Halo veterans know can sometime be a problem with alot of players out there.. "OMFG my KD went down a point my lyfe is fkin ovar!" :"(
But anyway I dream of the day we get truly dynamic objectives such as that.Also the Updated Elephant from I think the H4 campaign would be so amazing if they remade Sandtrap with those drivable for each team, where this "Mammoth" would be able to crush Spartans and Warthogs or other vehicles infortunate or foolish enough to get in its way; unlike the Elephant in Sandtrap that would get stuck on a dime.. Maybe im just a dreamer but I really dont think that this kind of dynamic objective is too far fetched to be pulled off in Epic fashion. I guarantee I personally would reserve my copy of H6 immediately after I saw that kind of gameplay even teased in a trailer. .
One last MINOR but cool thing I thought of for now would be the ability for atleast 1 human vehicle, maybe the Standard mongoose or Transport hog could have the ability to turbo periodically for some Epic moments like flying extra high off a ramp or splattering someone. They can have it balanced out where you cant turn sharply left or right while you turbo so once you hit the thrusters, you are committed to going in that direction for the duration of the boost.Just my 2 cents and I could of mentioned alot more things I would like to see changed as far as vehicle tweaks and balancing goes but I think its obvious to anyone who has played the last few Halos that the vehicle gameplay and fidelity in H5 are lacking in many departments while in a select few others 343 made welcome changes, I'll give them alot of credit for Warzone but I and many other Halo vehicle fanatics like me desire more of the classic vehicle gameplay that made H3 and Reach BTB so fun, intense and Unpredictable. Hopefully by the time H6 comes out 343 will have learned to strike that perfect balance with the vehicles and hopefully add even more variety to the vehicle lineup in Core Halo modes.
Thanks for reading my rant and my 2 cents on some of the things I would add or improve to Halo.