For me personally I always liked:
Maps:
Halo CE:
- Sidewinder - Rat Race
Halo 2: - Headlong - Relic - Waterworks - my favourite - Turf
Halo3: - Last Resort- The Pit
Like to mention also: - Valhalla - Infinity - High Ground - Terminal - Coagulation
- Death Island
Possible DLC maps maybe… 
I liked that you could once:
- Choose between an Elite or a Spartan - Dual Wield Weapons
- Use a Flame-thrower
- No bloom (bloom was a nice change in reach, but I do miss not having it)
- Post game party up System
- 1 - 50 ranking system
Halo: CE Anniversary will still be awesome just playing a remastered Halo CE campaign.
I was think that It would be cool to have individual Halo Playlists/game-type, for example ypu could have " Valhalla Halo CE", which would have the same “Valhalla” map but with only Halo CE features such as no armour abilities, or pick ups (e.g. bubble shield), just an idea, don’t hate me for it 