H2A definitely has the best engine we'll ever get. My favorite would be Halo 2, but it's far from perfect. I'd change a lot about H2 if I were designing a Halo.

: Of those, if MP play had absolutely frozen there, would you still be playing?

If you mean H3 played almost identical, then yeah. If you mean no Halos after H2, then yes but rarely. Only with friends.

: Maybe some new additions, like new modes or weapons or vehicles or maps,
: but nothing else (i.e. abilities, ordnance).

Absolutely, new modes, guns, aliens. All welcome. It's redesigning everything that's the problem. I think at this point the only Halo thing left is the AR. They even changed the Ghost!

: I ask because I fully agree with your take that sprint (faster move speed in
: general) has really wrecked Halo. I wonder all the time if they simply
: went back to, say, Halo 3 MP rule sets, plus fully-supported forge and
: theater, if the magic would come instantly back.

: Do things really have to change? Did Halo ever really need anything else?
: (I'm asking rhetorically)

I believe the core gameplay, art, and sound should remain mostly the same. There should only be changes where there are problems. Like adding boarding in H2 because H1 vehicles were OP. Stuff like that. Dual wielding was dumb, but it didn't have to be. Not messing with the damage of guns along with a better way to stop you from throwing nades or meleeing would have made dual wielding great.

I think innovation in Halo should be in the featureset. Things like Matchmaking in H2, Theater and Forge in H3. Rendered films, Bungie.net stats, etc. What more could they add? Just off the top of my head...

• Custom Games server browser. Battlefield has been doing server browsers on console for years now.
• A fully fledged PC based map editor for Multiplayer, Campaign, and Firefight. Fallout 4 is paving the way for mods on a console, Halo could use this. Some people don't like forge maps.
• Give the community access to Megalo (gametype scripting engine). We saw some great stuff out of the H4 modders. Official support would be great.
• Good spectator mode. H5's was pretty shitty. Might be better now idk.
• Copy Gears of War's Beast mode. How cool would it be to control a Hunter? A Grunt? Brutes, Drones, Jackals, etc.

I won't be able to enjoy 343's "Halo" because it doesn't look like Halo, doesn't sound like Halo, and now it doesn't even play like Halo.