I too am very excited about this...its one of those things that brings the open world to life. So a bunch of stuff comes to mind:

 First I'm thinking that there will be a combination of the two ways you laid forward. Side missions/quests to send your hired characters on & companions for battle. In a way we actually already have the companions thing in the investigation options,  those 3 IG you can select. 

Next, how about those "pets" we will be getting for buying in early. I kinda see them as being things like servo skulls, gun servitors and war dogs or other "service" animals/creatures.

Finally, the human retinue members can draw from all sorts of types, IG, assassins, bounty hunters, pyskers, tech priests, rogue traders, etc. Some useful for side missions/quests, others good as companions in battle.

Can't wait to see what they come up with.