That is, relationships with joinable NPCs.
The ideal of a non-romantic relationship which is still deeply developed, for me, was Dak'kon in Torment; since, by Chris Avellone's own words, we'll be seeing more developed NPCs as a whole as well as 'romantic' options, any preferences as to the sort of relationship you'd like to see developed?
For example, it seems pretty certain the Master-Apprentice angle will be there at the start, with you as the Apprentice.
What about a Master-Apprentice relationship with you as the Master?
What about a friendly rivalry with one of the NPCs, each of you trying to one-up the other?
What about a not-so-friendly rivalry, with the NPC constantly trying to outdo you and thus making you want to hack him into chunky giblets?
What about a character whom you can't fully rely on, like an Ex-Sith in a Jedi party who either helps or betrays you later on depending on how you treat him?
Any 'relationship' that appeals to you that doesn't involve painfully bad come-ons is fair game.