As I've said in other threads, Halo's MP worked best with a level playing field. That's what set it apart from other popular shooters, especially after COD went the custom loadouts + Skinner box route. Loadouts in Reach were a step in the wrong direction, but at least they were preset and everyone had equal access to them. But Halo 4 just went full on COD-style with them. Custom loadouts complete with two slots for "perks" and rank-based unlocks for weapons & abilities. Hell, even personal ordnance was very similar to COD's Care Package kill streak. That's not Halo. All they had to do was take away shields and add ADS and we'd have COD with aliens.

There's other ways to advance the series without ruining competitive balance. Invasion could be retooled to be a large-scale 16v16 gametype similar to what we see in Battlefield. Or even better, they could make the single-player levels a lot bigger, more open, and less linear. What set Halo 1 apart from other shooters of its day were its huge, open outdoors environments. However, level design in Halo has stagnated at Halo 1 levels and has even regressed in certain ways, with lots of arbitrary barriers to exploration; had Bungie gone overboard with invisible walls and kill barriers in Halo 1 & 2, we might never have been able to get to the bottom of the security substation on Silent Cartographer or explored the mountains of Delta Halo. Halo could set itself apart from other shooters by improving level designs and give everything the same epic scale and feel relative to the competition than we saw and felt after touching down on Halo back in 2001. Crytek tried this with Crysis back in 2007, though they too ended up making much more linear and constrained levels in the sequels.

Another way Halo can move forward is by making Forge a full-fledged map editor. I'm talking something really ambitious, not merely expanding budgets a bit and having a Forge World 2.0. I'm talking something that combines the best of Forge with the best of the Far Cry 2 & 3 map editor. I'm talking absolutely massive budgets and a variety of Forge pieces that dwarfs what we saw in Reach and Halo 4. I'm talking a terrain editor and the ability to set the environment type (desert, forest, swamp, arctic, etc.), weather, and time of day.

You want to evolve Halo? There's plenty of ways to do it without tampering with the core mechanics that make Halo what it is.