Titan

"Ensemble has been wanting to make an MMO for a long time. That was in production for a long time. The Halo IP was a great IP to launch an MMO with. Microsoft... hasn't had the best track record with those. And we worked on it for a long time--we had staffed up an almost-40 person team. And then there was some reorganization at Microsoft, and the new bosses thought it wasn't the best idea anymore. It had actually been green-lit, and then it got cancelled after that."

- Dave Pottinger

Titan was a codename for a cancelled Halo MMO that was in development in between 2004 and 2007 by Ensemble Studios. Following the project's cancellation in 2007, Ensemble Studios focused all of its resources into developing Halo Wars.

Overview
Titan was a prototype game developed to compete against Blizzard Entertainment's World of Warcraft. The project was internally cancelled in mid-late-2007 with no formal announcement of its development by Microsoft.

Around 40 staff in three teams worked on Titan whilst other Ensemble's staff focused on developing Halo Wars, though one team remained in the project six months after. The project drew heavy inspiration from World of Warcraft for the basic structure and style in the earlier stages of development. The project was said to be radically different in the later stages, exploring novel concepts and gameplay additions such as "heroic stylized artform" and cover system (currently used in Star Wars: The Old Republic), and public quests (currently used in Warhammer: Online). In addition, instead of mana-points as used in World of Warcraft, a character's special ability is referred as psion.

Features
The descriptions in the following sections are based on the gallery of images and, as such, might not be entirely accurate.

Classes
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Assault
Assault was intended to be a human class.
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Forerunner
Forerunners were intended to be a playable class.
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Marine
Marine was intended to be a human class.
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Mimic
A Mimic was an unknown female humanoid creature. Their name and concept art implies that the Mimic could shapeshift into various forms.
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PSI
PSIs were an intended class. It is unknown what their role was though they appear to be humans with some level of augmentations.
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Rogue
The Rogue was intended to be a human class. One may presume that these individuals were designed to have expertise in stealth combat.
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Spy
The Spy was intended to be a human class. One may presume that these individuals were designed to have expertise in espionage.
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Species
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Demons
Demons were an unknown alien race.
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Diggerz
Diggerz were an extra-terrestrial animal species. It appeared to be an armored, bioluminescent biped with clawed forearms that served to improve digging capabilities, hence its name.
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Jaggmaw Sailtail
The Jaggmaw Sailtail was an animal used for transporting characters around on their backs. It bore a notable physical resemblance to the Blind Wolf, a species that was cut from Halo: Combat Evolved.
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Mob
The Mob appear to have been intended to be a new species. The creature bears a strong resemblance to Earth's boars.
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Sentients
The Sentients appear to have been a new species. They appear to have been a humanoid species with mechanical attributes, such as reverse jointed limbs and visors for faces.
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Sigma
Sigmas were apparently a new species of extraterrestrials that were intended to appear in the cancelled Halo MMO. Some of the Sigmas seen in concept art are decidedly human in appearance, while others have more distinct alien features. Not much is known about them other than the male and female genders each had a distinctive physical appearance.
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Vehicles
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Aardvark
The Aardvark was a human vehicle intended to be featured in the cancelled Halo MMO. It would have possibly be used as a reconnaissance vehicle similar to the M12 Warthog, but with an appearance more similar to the Wolverine.
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Warthog
"2 Tickets to the Gun Show" was the developer name of a vehicle intended for inclusion in the untitled Halo MMO. A variant of the Warthog, it was apparently to be used by law enforcement, as evidenced by the word "POLICE" printed backwards on the hood of the vehicle. As can be seen in the concept art, there appear to be two seats mounted on the back of the vehicle, each with its own turret. This brings the passenger capacity of "2 Tickets to the Gun Show" to four.
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Grackle
The Grackle was a human motorcycle, which was going to be featured in the untitled Halo MMO
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Rhino
The Rhino was a human vehicle, intended to be featured in the canceled Halo MMO. It was intended to be used in the role as a gunship. It bears a notable resemblance to the Pelican dropship series, though with added missiles, chainguns, a thruster over its tail section, and enlarged wings with built-in ducted-air fans. It may have been the conceptual inspiration of the Sparrowhawk, a craft that would have filled a similar role in Halo Wars.
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Runner-class Transport
The Runner-class was a Covenant Naval vessel designed to transport the Hierarchs, known to be featured in the cancelled Halo MMO. One such Runner-class ship was known as the Throne Warden. It was completely manned by the Sangheili, and had a limited planet-to-planet travel distance, with no system-to-system slipspace drives.
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Scorpion tanks
Two Scorpion tank variants were intended for the cancelled Halo MMO. Both variants had more of a stylized design that caused the tank to closer resemble the animal from which the tank's moniker derives. The "Lash" Scorpion sports a brown/yellow color scheme and a longer "tail" whereas the "Venom" Scorpion sports a grey/blue color scheme and a shorter "tail".
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Forerunner combat armor
The Forerunner combat armor was an armored suit in the cancelled Halo MMO. The armor was designed for Forerunner use but due to humans and Forerunners being the same in the MMO, the armor could also be worn by humans.

MJOLNIR combat armor
These unknown MJOLNIR armor models were variations of MJOLNIR Powered Assault Armor for Spartans in the cancelled Halo MMO. Not much is known about the armors, although most superficially resemble the Mark VI, except for the shoulder pauldrons. Several variations were designed, one of which had a chainsaw in place of the left hand. Unlike the armors seen in the actual games, most of these designs are quite extreme and heavily stylized.