Imagine this, after the events of Halo 4-6, you can finally have a chance to make your own hero, like in Halo Reach. Whether it be a UNSC Soldier, ODST, Spartan, or even a Grunt, you get the chance to explore the Halo universe like never before, and your the main character. Now I know that the Halo MMO was rumored to be cancelled, but I believe that after the Reclaimer Trilogy is finished (Halo 4-6), 343 Industries will reboot the Halo MMO project, so that way you can continue Halo's epic adventures, and create your own!
          I'm rather sure that the MMO was in it's starting stages around the time Halo 2 was being created. Bungie did say that there was content cut from the game because they were "rushed" by Microsoft to get it done. I have come to believe that perhaps it was also because they were doing the Halo MMO as a side project. Now for the reason of it's cancellation, could have been because they did not want to repeat the same scenario with Halo 3. This is why I think 343 may wait till Halo 6 to pick the project back up again.
        Now then, from the concept art and renders I have seen, the game looks like it would operate like World of Warcraft or other notorious MMO's .

Gameplay HUD.

    Other concept art, and in game pictures go to show that the game will include vehicles which will most likely serve as mounts, and other heavier vehicles which may serve as either combat vehicles, for PvP or for Instances (Dungeons),or they may serve as a taxi type of vehicle for players who cannot afford mounts, or are to low level for one. Here are a few examples of what these mounts could be. Also you may see that from the looks of these vehicles, the UNSC and Covenant have upgraded their technology, which is why I believe that the Halo MMO will take place after the events of Halo 6. Now a majority of this art has Ensemble Studios trademark on it, they were also responsible for working along side with 343 Industries for Halo Wars.

UNSC Vehicles:

Possible UNSC faction mount.

A higher level or "taxi" mount.

End game or PvP battle mounts.

Covenant Vehicles/Mounts:

Possible Brute racial mounts.

Covenant dropship, taxi, or PvP based mount.

A new age Covenant dropship.

Another unknown Covenant ship.

 Possible Forerunner Vehicles:

Forerunner vehicle.

Different variations of the Forerunner vehicle.

     Now the Environment concept art of the game looks absolutely amazing. From the amount of work they put into these pictures, I believe that 343 Industries is simply putting this game on hold till Halo 6.

Human controlled city.

Rock formations.

Possible Forerunner ruins on a unknown planet.

        Other screenshots, such as the ones below, show that the game was in working order and that it had been well into it's development phase. Here are some shots of what the in game graphics and environments look like. Now the graphics look better than Halo Combat Evolved, which for a MMO, is pretty good.

Jungle environment.

Morning jungle environment. Note the Human looking structure in the upper right corner.

Another jungle shot.

And another.

Two destroyed UNSC Warthogs, with marine casualties to accompany them.

A canyon/desert environment. Notice the Halo Ring arching upward.

Another picture of the destroyed Warthogs, with strange crystal formations around them. These crystals could be possible crafting materials.

      Here are a few shots that I believe are of a Forerunner city, as the structures resemble much of the Forerunners Architect. One thing I noticed in the pictures was the presence of humans in some of the shots, possibly hinting to humans inheriting old Forerunner tech and planets.:

Far view of the a possible Forerunner city.

Closer view of a section of the city.

A lake side view of the city, note the ships in the sky, which shows that this city has inhabitants.

I am guessing that this is some sort of park, it is interesting as to how there appears to be human civilians walking about.

A bridge that probably goes from the city, and into the wilderness. Notice the human figures standing on the bridge.

A possible outpost or perhaps a type of housing area. Note the figure, wearing hiking gear.

        By the looks of it Forerunners are going to be a big influence in the game, as they have in all the others. Now as to whether Forerunners will make appearances or be a playable race, is uncertain, for now. However there are pictures of multiple new races that have been never before seen in the Halo universe. As to whether they will be civilized factions you can join, random enemies spread out across the galaxy, or even playable races, is unknown. The Covenant has appeared to still be holding together as well, including Elites and Arbiters in their military.The classic races are still there, such as Jiralhanae (Brutes), Unggoy (Grunts), Sangheili (Elites), and of course the Flood. As for San 'Shyuum (Prophets), Mgalekgolo (Hunters), Drones or Buggers (Yanme'e), Skirmishers/Jackals (Kig-Yar), and Engineers (Huragok), no concept art has been seen for them, yet. Here is concept of the classics, and some new and interesting species as well.:

A group of Covenant soldiers, displaying a Elite, Grunt, and two Brutes.

A height line up of several species in game.

One of the new unknown species.

A smaller "engineering" species.

And another.

Possible wilderness beast.

Another beast.

Possible Forerunners.

Another unknown humanoid.

Another unknown species, notice the different variations of it.

A picture showing a human becoming the host of a Flood infection form.

A new Flood form, or a new threat?

 
         Now as for the story line of the game, no one can really make any predictions until we get a idea of what's going on in the Halo 4 story line. As I stated earlier though, I do think that this game will be made to pick up were the Halo story ends when Halo 6 comes out. As for what it will be played on, I guarantee it will be a PC game only, as the mechanics for advanced MMORPG's cannot be played on something like a Xbox 360 console.

        As for information about level caps, gear, classes, etc. there are also pictures of that as well. The level cap for the base game seems to be level 60. As for classes, the only two I have seen so far are human based, those classes being Marine, and Psion. There is one picture that shows the progression of a "Assault class" which I believe is possibly a specialization that you can choose for the Marine class, or vise verse. Other pictures also suggest that there are hunter, caster, tank, melee, and healing classes. Here are some photos of the currently known classes.:

The level progression of the Assault class.

Armor Progression of the Psion and Marine class.

       And here are a few pictures that hint at possible classes.:

A example of a possible hunter based class.

Example of a possible healing based class.

A example of a possible rogue based class.

A example of a tank based classed.

A example of a melee and casting based classes

          Final Note:

          For those of you who have not noticed yet, it is very evident in the pictures that the Humans/UNSC have gotten some upgrades with weaponry and armor. As I said before, this is why I think that the MMO will take place after Halo 6, because during the Reclaimer story line, the UNSC is going to run into the Forerunners, and our newest enemy the Prometheans, and likely "Inherit" their technology. Till then though, we have much to look forward to with Halo 4-6. Thanks for reading this, and good gaming to you all! Feel free to add comments and your thoughts on this subject.
        

 New Information as of 7/14/12:
        Here is a link to were I found most of these photo's. You will find all the pictures above and more from the Halo MMO in this link. Enjoy!
 http://halo.heavengames.com/gallery/view_album.php?set_albumName=halo_mmo


Additional Information: If the link won't work via click, try copying and pasting it in your search bar instead.

New Information as of 7/17/12:
        I have just learned that there is currently a Halo MMO in production called Halo: Galaxy of Combat. The game is being developed by Bay View Game Studios as of now and they are licensed with Microsoft and follow their strict guidelines to making fan made games, no info nor pictures have been leaked yet other than the fact that the MMO is free.
Here is a link to their homepage:
http://www.bvgstudios.net/