The "holy grail" for an IP like Halo at this point is a robust MMO.  Here’s my suspicion, Bungie and M$ are working on a MMO FPS for Halo, and Reach will actually serve as the test bed to work on some of the ideas that will be incorporated into the game.  This makes a lot of sense for the two companies, since from an execution standpoint, this allows a sustainable model for both companies to work together in the current 2^nd party format.

The cost side of  World of Warcraft is approx $200-250 million, but with a top subscription cost of $14.99, and with the membership levels at 12 million, the ROI for revenue has been over 10X, and that’s just staggering.  If 360 continues on it’s current LTD trajectory, it is going to cross 50 million consoles with little problem.  The question is whether a strong MMO on the 360 would serve as the perfect relay hand-off to Xbox next, or as a hindrance.  I think it would aid the transition since you would be able to upgrade the client to run at a superior level on the next console, and also have it as a “Wii Sports” type of launch title. 

This would also allow them to spin-off things under Gold that really causes a lot of issues with customers/observers.   You can pay an increase annual fee for Gold or pay monthly for the MMOs (Halo, FF14, 1vs100).  343 would maintain the game with technical support, with Bungie working on the 18-24 month refresh of new content with expansion packs.

As for the design of the game, the Halo-verse is ripped for the execution of a MMO.  On the human side, Spartans will be the uber characters as tanks/DPS/soloable characters…ala Death Knights in WoW.  They are self-healing, and able to melee and range, but are not so overpowered  that everybody chooses only them.  Of course, you’ve got the ODST to server as the rogue/stealth class with sniper rifles and the sort.  Then, you’ve got the medics to server as buffers and healers, and that’s pretty common.  Lastly, you’ve got the Engineer as a combat buffer and pet handler.

You can look at the Covenant to see how they too can be classed out.  Brutes as tanks…not like Spartans of course, but rock hard.  You’ve got the Elites as stealth/rogues and more capable…not quite up to full tank strength, but more like the Shamans of WoW...can buff, but also can fight solo.  You’ve got the Jackals as your hunter class, and maybe have a pet for them of some kind.  The grunts can be a support class, and maybe a cross between healers/buffers.  So the classes for the Covenant wouldn’t Mirror the UNSC, but would in the total pool of functions. 

The Spartans would of course be the most popular characters, but I think people would still try the other classes, and while the game would be heavily based on a player’s ability to shoot FPS style, you would need to level and get better gears and stats to be able to become deadlier.  The game would really allow for the “soft landing” from the 360 the Xbox next, and it makes tons of sense for Bungie to establish a steady source of revenue and income t o allow them more flexibility to fund other projects.  For M$, it’s a no brainer as you’d probably see 3-4 million subscribers to the game, and would allow more flexibility to innovate with XBL.  Anyway, these are just my ravings and probably isn’t ever going to happen, if for nothing else…for the risk of failure to a brand that has seen nothing but success since being launched.

interesting theory..

Yeah, I think the only complicated issue will be the give-n-take of XBL. There is no way you're going to sell people on the concept of paying for Gold, in it's current format, and then charging for a Halo MMO. If you look at Dust 514, I think M$/Bungie could study that very carefully to see how to technically implemented, as I think CCP has one of the best solutions in Eve.

Do you mean Trojan Horse?

No, I meant House...like the House that Ruth built...errrrr....damn you caught my mistake...darn non-modifying thread titles...lol!

Lol, epic. 

So the idea of HALO FPS MMO seems plausible, but many factors working against this happening. Bungie no longer wants to do Halo, correct? M$ would have to charge a separate monthly like in FFXI, this doesn't seem to be what M$ wants. They want everyone to get gold. Otherwise, good idea i just hope its full action like in Borderlands.

Halo MMO sounds good. I think that PvP would be most popular, but battlegrounds would just be exactly like playing Halo online, except dealing with certain people dealing more damage.

This kind of things gets me day dreaming...
How to handle healing by medics and such? It would be too weird to have some kind of healing gun. and it would be awkward/odd to select people to heal. I was thinking the game would be heavily based around combat. Therefore, medics still fight. Then the medics have a buffer range around them, 20ft (or whatever) in which they heal their teammates. Medics do less damage and are invulnerable to headshots.

I have more ideas but I have class now.

Trojan House LOL.

And I disagree with your theory. Halo doesn't suit well for a MMO, and even if it did, what does that have to do with Halo: Reach?

http://www.thecoolhunter.net/architecture/Trojan-House---Melbourne/

The only problem I see with this is the remaining lifespan of the 360.  An MMO needs a large, active userbase to sustain the investment.  Last year would've been the ideal year to release an MMO on the 360, as that would allow a good 5 years before the next console releases (speculation, of course).  I also speculate that the premature cancellation of True Fantasy Live had to do with this very timetable.

A successful console MMO will be very difficult to pull off, as a lot of factors have to be drawn together with perfect timing and a sustainable product.  If anything, maybe MGS and other publishers are looking at ways to shake up the current MMO structure to make it more sustainable for consoles.  Regardless, your idea has merit and likely has been discussed before by the powers that be.