In a dream scenario I would treat Halo 5 and Infinite like they didnt happen and restart the story from the end of Halo 4, trying to actually achieve the interesting promise of that story.
Im one of the folks who believes that Halo 4 was actually an incredible opening to the trilogy and a great start for a new interesting path for Halo, Halo 4s campaign ruled, its story is potentially the best in the series, I loved the Promethians here, the new promethian weapons were mostly sick, the music was incredibly and new, the variety in the canpaign was so well done as well and it sets up so much for the sequels to play with.
Not to mention alot of the MP maps were really cool, the main weakness of the game does come from its multiplayer, though. The weapons drops being random were anti competitive and the ranking system was terrible, as a competive halo game it was bad but like Halo CE which I would say wasnt very well balanced this was thier first Halo game, I was just glad it was so damn fun and felt like Halo (even with sprint since the map sizes/radar were increased to compensate), I didnt complain about the loadout system because it honesty didnt make much of a difference, luckily it was undertuned and very safe so it didnt make Halo feel like something else. (Although the random weapon drops sucked), casually in Big Team Batlle it was very fun.
I would try to emulate in a redone Halo 5 something similar to the vibe of that teaser in the desert with Master Chief alone searching for answers to save cortana. As for how multiplayer would play I would kill loadouts/random drops from 4, and make abilities like the dash or ground pound etc. pickups you have to earn/control on the map with large cooldowns with thier ability slot giving the player a distinct color around apart of thier body so you know what ability they have when you see them, the sparton weapon drops would stay but they would be different on a per map basis with a repeatable setup (spartin dropping would be on a team basis and inform everyone on the map on where it was going to drop and would lock the opponent out of dropping while your team drop is in progress with a 10 second countdown before the drop), I would also lean way harder into intetactive environmental stuff that can change things drastically on the map, the radar would be like the old school one aka useful, one thing is for certain I would never sacrifice Halos speed/pace/feel to appeal to modern gamers or fans of other games only trying things that would make sense.
Halo 6 for the story it would have Cortana be saved somehow around the midpoint through halseys solution where they pull a reverse promethian and make cortana human essentially to defeat rampancy, the whole forunner thing would get wrapped up with humanity becoming more evolved or a path for them too, chief would probably die at the end though (or be black holed somewhere alone who knows). Gameplay wise the bigger changes would be more slots up to three now you can hold for Abilities to be picked up/controlled on the map but this time there are limited uses until they break and have to respawn. The biggest change to MP sneak peaked from my environmental interactively I introduced in 5 would be at certain times of the match major events occur changing the map for the rest of the game creating different strategies for different times of the match.
(Also in this dream scenario Master Chief is in smash and the ranked syatem would be a better version of Halo 3s with all of its great game modes)
Realistically what I would try with the franchise in a non dream scenario is I would reboot it and move completely away from the main plot, I would make a trilogy of games based around the Arbiter as the main character Halo: Arbiter or something like that. There would atill be humans weapons but they would be rarer since there wouldn't be very many humans in the game.
What actually is gonna happen is we will receive crap, unless maybe a good Halo 3 remake is in the works, that could be dope.
