User blog comment:Nord Ronnoc/Mass Effect San Diego Comic Con - What We Know So Far/@comment-4452849-20140726222529/@comment-24200891-20140727162812

It's spin off because it's in the same universe and there is line "You will see a bit of old with the new (old characters MAY appear) !!". If it was a reboot, Shepard would again be a protagonist, in all movie reboots were same main characters.