They need to stop grinding out titles like the next installment of Madden or Guitar Hero and put more thought into them. In short - more exploration, more storytelling
through discovery instead of narration - make things new again and stop just putting me inside a suit of power armour and shoving a rifle in my hand because the game case says Halo on it somewhere.
They tried RTS with Halo Wars and even though I'm a fan of the Halo-verse I was pretty lukewarm about it.
For me ODST was the most original of all the titles (not counting Halo Wars) since you don't play as a Spartan. I loved the storytelling in ODST and it shows how much Bungie learned over the years and the tactical style of combat was much better than the typical run & gun in other installments of the fanchise. If it had multiplayer co-op I'd say it was a better game than Reach.
Then there's the one chapter in Reach that
every single player I've talked to absolutely loved...piloting the Sabre. It's been a long time since I've played Wing Commander but if Bungie wasn't actively channeling Chris Roberts then they were at least being subliminally influenced by him - the space fighter portion deserves it's own game. If 343 wanted make it it's own sub-franchise that's possible too (and without streching the ip as far as they did in Halo Wars). Run & gun furball battles between fighters in space, close air support for troops while flying a Hornet/Falcon/Vulture, troop insertion with a Pelican (maybe a stealthy spec-ops insertion for added difficulty) and even sections of slow, plodding RTS (or even turn based) maneuvering of capital ships providing fire support & planetary siege/defense.
How about letting me explore more of the Halo-verse as a civilian? Put the fear of the covies back into people by stripping them out of the armour and maybe having them fight off grunts (or the Flood!) with a pistol. If you want to see players cower in fear again...don't make the enemies stronger, make the player
weaker. How about fighting off a horde of enemies with no weapon other than a vehicle and a lugwrench.
I'm through doing 343's homework, they want more ideas then they can put me on the payroll.