Ensemble Studios is currently at work on the Halo themed strategy game however there was also a unannounced Halo MMO that was in the pipeline from the studio.
The game was not cancelled recently as the title was a prototype worked on by the studio during 2006-2007. Ensemble was working on console and MMO games at the time but no details were released.
Screenshots of the MMO project have surfaced showing in-game mockups that more than bear a resemble the popular MMORPG from Blizzard, World of Warcraft.
Find concept art of the Halo MMO here however we managed to upload the in-game screens found after the jump!
