I think Potomac Yards would alienate fans north of the city. It's not that bad of a shot by metro down the green to the yellow, about 45 minutes to Wheaton with reasonable connections, but it's closer to an hour to Bethesda.
The area between East Glebe and 4 mile run (essentially the strip mall, PY station, and the VA Tech building) would need to be substantially reimagined to add in more restaurants for the arena not to be sort of isolated. A portion of the mall near VA tech was supposed to be incubator/industrial park space for startups. The best space left is mostly where the viable stores are (Target, Best Buy, Petsmart . . .), plus there's one block near the condos and town houses and the station. I'll assume that's the land they are eyeing, but the streetscape is not good for an arena there. There's two office buildings on the opposite side of East Glebe from that block.