Keep a real classic playlist option available. Do like Reach, but have Evolved and Classic versions of everything. That way, 343 can evolve how they want to, while a real classic option still exists that I don't have to hope for. I mean, I have to hope for it to show up, then I have to hope it gets enough votes...
I prefer Infinity Slayer to Legendary Slayer, but a bigger focus on the core Halo feel would bring back those old fans while the evolved game would bring in more.
Next, lets keep some of dat music from Halo 4. It grew on me after a while, and I feel badass while it plays on SpOps. But, include more classic music as well. Halo 4 was too much new. I still boot up Halo 3 for some music now and then.
As for armor, I liked Halo 4's fully customizable Spartans, but Reach's modules and bits and pieces were fun too. Merge those systems 343, and you'll have a badass Armor system.
Finally, don't over complicate the game. I liked Halo because I didn't have to choose guns and perks and crap for my given situation. If I want to snipe in Halo 3, I just run for it. In Halo 4, I have to build a damn Loadout for it to be effective. Now, I like choosing between a balanced set of primaries and AA's, but adding anything else is just bringing in too much for the simple game that was Halo. Halo was great because it wasn't required to build a loadout in order to be competitive. I feel that these complicated loadouts have broken a good portion of the core Halo feel for me.
I loved Halo 4, and I plan to have Halo 5, but the complexity of this game is too much at this point. However, I can put up with all of these loadouts if we have more focus on the core Halo gameplay that has been lying dormant for years, now.