Why would you want that? Halo 3 already has good graphics and if they do that then the only way I'd get it is if they add something else to the table like spawnable AI in Forge. That is the only way it could compete with the revolutionary Halo 2 Anniversary. When I say AI in Forge I mean every AI in forge from ODST to Elite Majors and Brute Chieftains. You also need to be able to pick which weapons they use in battle but, the UNSC need to have all UNSC weapons as spawn options. The covenant need all covenant weapons as options (except only spikers, hammers and maulers) for brutes.
These AI need to be the exact same as the ones in gameplay and they could either make the friendly AI follow you or just hold their position. This is definitely possible on console because Star Wars Jedi Outcast and Academy did it and they were 2002-2003 games and don't give me the excuse "well those AI were simple". Many of the AI in Jedi Academy (jedi, reborn, cultist, rancor, hazard trooper, sentry, saber droids, noghri, probe droids, and saboteurs) and even Jedi Outcast (good soldier AI, reborn, jedi, the imperial construction walkers, and galak_mech) had some AI that was on par with or better than the halo 3 AI.
The number limitations should be the same as they are in campaign. There should at least be 40 AI max (because once in Halo 3 I had over 30 marines engage about 10 covenant); but thats a modest number because I expect there to be about 64 AI as the max limit for Halo 3 AI at a time in one map. Every type of marine AI needs to be spawnable (from African male, to female, and all the other males), heck Jedi Academy did that with the jedi and reborn AI. The AI added in Forge have to be saveable with the map so, that when a player saves a custom map that player can have their own custom missions (I will say it again Jedi Academy and Outcast allowed it). Since Halo 3 Anniversary will be on xbox one the console should be powerful enough to handle it.
If E43 really wants to treat the fans then they could add heroes and villains as AI (Sergeant Johnson, Arbiter, Miranda Keys, and John 117) even if they have infinite health. They could also add a coupple extra units like Zealots, Ultras, and maybe even spartans which could just be marines with shields and spartan skins (Star Wars Jedi Academy did it by adding many extra jedi and sith units for fun and Star Wars Jedi Outcast even did it by adding female jedi as an extra AI).