The strongholds in BG2 were quite popular. They were basically side quests where you did a job related to your class skills. If I recall correctly multi-class and dual-class characters could pick which of the stronghold quest chains related to their classes to do, but they could only do one (I am sure that there were mods that relaxed that constraint).
The strongholds did not replace the common gathering places for your companions, which was the Copper Coronet for most of them, though some of them would head to other favorite places when not in your party.
In any case, I imagine that they would be quite a popular feature if they were added in to BG3 in more or less the same way. It would be a nice nod at the history of the franchise, and would probably be fun for new players as well. The quest lines tended to be fairly short, so I doubt that it would ad massively to the dev time either.
