Personal Top 5 for a successful relaunch in order of importance:
1) Fixes to make the ports of each game play 1:1 or as close as possible their original incarnation. (H1 Pistol, H3 Hit Detection, Maps not being the latest version as their previous generation counterpart, etc.). Gameplay should always come first.
2) It's been said a million times, but a new UI. I think it should be in the style of Halo Reach or Halo 3. After all, this is a celebration of previous Halo titles.
3) Bring back the veto system. Or drastically alter the voting system. For a game with hundreds of multi-player maps across 5 games, I sure do play the same maps and gametypes quite often.
4) Expanded Post-Game Lobbies and Carnage Report. Have the option to stay with the current lobby of players and be able to talk to the other team when a match ends. Basic details like which game, map, and gametype need to be included in the post-game carnage report.
5) Each game should have its original set of armors, with fully customizable emblems and armor for each separate game. The game is missing player originality.