Recently I came across an article about possible new Fallout game ideas, and after reading for a while I found one that really got me thinking, Fallout Legion. An entire game based around the main character becoming part of the Legion and working up through the ranks, becoming most anything they want, a great general, Caesar himself, a spymaster, the list goes on. Alternately, the player could join the slaves of the Legion, quietly living as a slave of the Legion, day to day making allies and enemies and possibly one day starting a rebellion. Or they could escape the Legion and strike out into the wastes that the Legion holds, doing as they wish.
Having a game based around a "bad" canon ending could be refreshing for the series as a whole, who says every amazing person in the wastes is a good person? Why not have a protagonist that helps rebuild the broken Legion after the events of Fallout New Vegas? Though they lost the fight for the West, the Legion lost no territory in its war against the NCR, so in theory the Legion still stands, albeit weaker than before. That's where the player comes in, working their way up from a lowly slave, to a great legionary, or even Caesar. Not to say that the player couldn't change the Legions direction, possibly changing the ideology of the Legion to become more tolerant and overall a good thing for the wastes, even if they are a bit harsh in their methods. Raul from Fallout New Vegas even said that the Legion helped calm down the areas they control, getting rid of raiders and making travel safe.