I also started this thread with the assumption that Pens owners would own the NBA team
That felt a lot more possible when ownership was relatively local. Fenway's strategy seems to be based around purchasing already successful franchises/clubs. There were plenty of cheaper options than the Penguins or Liverpool, but those are 'global' brands (to be a bit generous to the Penguins and the non-global sport they play). An expansion team would be starting from scratch in a smaller market that has plenty of other options.
Fenway doesn't own the arena, fwiw, so it wouldn't be a situation where they'd be adding more dates to an arena they control. The arena doesn't need another anchor tenant to be profitable, either. There's as many non-hockey events there per month as the Igloo got in a good year. Hell, next month there's a stretch with 8 events in 9 days with only 3 of them being hockey.
If Fenway goes for an NBA team and it's not Boston, I can't say I'd have a clue who it'd be...but it'd almost certainly be one with a more global reach than, say the Sacramento Kings or an expansion team. The Bulls seem like the kind of franchise they'd be most interested in acquiring based on their track record, not that I think that's going to happen.