How many times has this idea passed through my mind?
Only a couple of thousand XD
Alright, serious time. First of all, is there a market for this a halo game of this kind? Halo is a shooter series, and the attempt to branch out from that into another was a failure. I'm looking at you halo wars... (I enjoyed it, but there was so many problems with it.) I don't think this could ever happen because of this. I wish it would, I'd buy it, but I don't think others would.
A halo RPG or MMO rpg would be really great. I mean, the chief is cool and all, but making our own characters and thrusting them into the universe would be great. Personally I would lean more to it being an MMORPG. Whichever it is, I want LOTS of customization options. Nothing is more annoying than walking up to a character to find they look EXACTLY the same as your character.
I think an Halo MMO, with large amounts of players waging war across the galaxy, would be epic. I think the time period most suitable for this is the human-covenant war as it is the time period that the most is known about and which is therefore easiest to do. Upon entering character creation you decide whether you are fighting for the UNSC or for the covenant. The majority of the game is meant to be player controlled, that is, the officers are players, the soldiers are players. These a supplemented by NPC's where required, but I think having a game where the players are in control of the factions is pretty cool. You are meant to obey the characters that are higher ranked than you, not doing this may result in a 'court martial' (Or an 'inquisition', for the covenant) in which your character might get demoted from their position.
Personally, I wouldn't like another 'world of warcraft' combat clone. Have halo how is should be, still a first person shooter. Level ups don't make you 'invulnerable master of doom' but rather allow you to pick a perk from a skill tree which slightly enhances your characters abilities. Example:
Race: Human (Spartan)
Perk: Enhanced speed
Description: They've always been faster
Effect: Your movement speed is increased by 10%
Or
Race: Brute
Perk: Blood lust
Description: Grind them into dust!
Effect: Boost to berserker rage. Increased your berserk movement speed by 5% and your berserk melee damage by 10%
See? A boost which gives them an advantage, but doesn't make them bad ass just for leveling up. Therefore, people who are skilled at the game will still be able to beat higher levels if that player isn't skilled.
A single player halo RPG would be cool too. Like mass effect and halo put together (Result: Awesomeness).