So the issue with this, unlike 4-team groups in Champions League (who play home-and-homes, 6x games) or the World Cup (all games at neutral venues), is that half the teams will end up playing 2 home / 1 road, and the other half will play 1 home / 2 road games.
That's why the 5-team groups are somewhat genius, because you play each of the other teams once, and it's 2 home / 2 road in all cases. I think CONCACAF came up with that for some world cup qualifying early rounds a few years ago, and I've suddenly seen it used elsewhere in other tournaments since - perhaps here most prominently.
I suppose you could also do 7-team groups, playing 3 home / 3 away, 2 groups per conference, with 1 team in each conference getting a bye to the knockouts (or do NBA champs / defending Cup champs, like you said - which might make a mixed-conference group, if both of those bye teams are from the same conference, but whatever, no biggie). Then you'd have a lot of choice of structure, over who you send through to the knockouts, and more room for a team that took an oddball loss to make up for it in subsequent games. Normally that would be a nuts suggestion, 7-team groups... but since the group-stage games are just regular-season games with added juice, they're gonna be playing them anyway, so who cares.