Want me to spoil the plot of almost every companion story other than your first one?
Here it goes:
"So, I knew this guy from my past"
"So, I found that guy. I'm going to go get him. Oh, no, you can't come. See you later"
"I finished my business with that guy in my past. Now you can forget I was ever here."
Just about every companion plays out like this. Granted, there are a lot of gems buried in there, but the formula is very predictable. Would it be too much to ask if all companions had missions like your first one? That, to me, is a lot more amusing than hanging around the ship and listening to your companions describe what they did. It might not seem like much, but going to Dromund Kaas for a hunting trip with my companion is a lot more engaging than hearing him describe how he thinks it'll go, then him saying "well, yeah, it went how I thought it would". I don't mind flying to some place I haven't been to in awhile to change thinks up. Take us off the rails a bit so the world can feel dynamic.
And then we have main companion storylines. I'd say that Vette had a very good one. Mako
Hopefully, though, all companion stories will continue in patches and expansions, etc. As it stands, all companions basically end their stories on "well, can't wait to see what the future holds". That, and Bowdaar