What would I do to Halo now?
Simple.

Ruin the franchise until it has no value as an IP - Get the original devs back together, have them pool money or crowd source it - buy the IP back.

Have Joe Staten fix what's been done. Or at least do damage control.
Which is what I was trying to do before. With no luck.

Gameplay?

Well, I would SLOW it down.

This is where they went wrong.
Slower games force you to think about your moves, and be accurate with every decision you make. Look at counter strike, STILL thriving almost what? 20 years later?

It's slow and methodical.

Think
Move
Action
Reaction

They decided to SPEED it up, so the minds of children (who are almost all affected with some type of ADHD, so take the 5 minute attention span of a normal human and cut that into half)
Would be stuck on all the pretty colors and action!

No more thinking was really needed.
No more timed melees! - Just press the button and you FLY at them.
The Energy sword could have been so much more - a SPECIAL Melee item that replaces your standard melee with an instant kill - 4 or 5 charges.

A counter ability which is EXTREMELY timed, someone melees you - pressing the button at the EXACT moment will COUNTER it and deal double damage.

A hard move - but an equally good reward. Imagine the skilled duels.

Plasma rifle would NOT have turned into the useless AR clone (Repeater)

It would eat shields, AND health - as it is highly advanced.
Slower fire rate and quicker burn out. Maintains "Stun" ability from the first game.

Plasma pistol?
Deagle from CS

Overcharge would eat shield - burnout
Overcharge again would take you down to one bar of health.

This beam did NOTHING except deplete shields in Halo 2 -

Use the LORE - it's a GIANT BALL OF IONIZED GAS.
It should be DEADLY.

Shields?

Have the pack from the Mark V located on the back, and weak to armour penetrating rounds (sniper, mag etc...) Randomized variable on "Critical malfunction" - shield pack is damaged, you have ONE remaining shield bar left, because the pack cannot refill.

Optional - "Critical Overload" (Lucy? From First strike. Self detonate when cornered. A la predator - would provide epic last stands, and tactical options. A needed sacrifice for team victory in MLG games. Could you imagine seeing a pro being cornered with his shield broken, and having to choose to self detonate to save his other mates?) People would eat it up.

Guns like the Brute shot - amazing.
The geometric values needed to think about where your shot is going added so much fun.

Fuel rod?
Pick it up in multiplayer - if killed, self detonate.
If out of ammo, self detonate.

Flame thrower- Nuf' said.

Boarding?
Ability to counter the board.

Also, ability to kick player off before the nade is dropped in your tank -
(Look up videos from the old game, Mech assault 2 - Xbox 1. They had a hacking mech that could attach to yours to boot you out of the cockpit - but you could "Counter hack" and possibly win - causing them damage)

The player should ALWAYS have an option. A way to win. Always.

Never ever take their hope away from them. However slim.

Vehicles?
Location based - effectual damage.

AP round in banshee gravity pod?
Sorry, that wing is gone.- you're sagging to one side.
Next wing? - flightless. Have fun with the landing. (again, not dead - just broken. Maybe bailing at the right time would save you. EPIC plays in MLG.)

The vehicles should have locations COINCIDING with the LORE.

Just like the ghost cannons in Halo 2 -

Banshee pods - flight
tail fins - stearing
Small space underneath - power junction
Cannons - obvs.

Now the other stuff.

The ability powers are in the game. Going to have to keep them now.

However, make them a utility.

They burn out after uses. This makes the player have to think about when to use them
Ala' flash nades, smoke nades and incins from CS - one of each baby, good luck. Use your brain.

No more mindless dashing. Pick your battles.

Teach your fans to think. To learn. To adapt and overcome.

Not to be mindless.