Both Kasdan and Abrams confirmed that their only criteria for naming characters was whether they liked the name or not. Seemingly, they liked BB-8 a lot:
"A lot of names came and went, and some stuck. I remember when we put down BB-8, it was the first and only name that droid had. Rey, Finn, and Poe went through many iterations. Kylo Ren and Maz Kanata were there very early on."
In terms of designs, a lot of work went into Kylo Ren's look, which at one point almost had him playing the role of the chrome trooper:
"The most difficult was Kylo Ren. I don't know how many hundreds, probably thousands of iterations. During the process, the costume for Captain Phasma was designed, pitched as one for Kylo Ren! For story reasons that didn't work, but she became one of my favorite characters in the movie."
Incidentally, John Boyega has his theories on who will steal the movie:
"It has a message of courage, of friendship, and loyalty. I watched the movie with Kathy last week, and I really related to Rey more than any of the characters! It's inspirational to me. But all the kids will concentrate on is BB-8."
The droid is clearly an important figure, and it seems there will be an element of bromance between him and Poe Dameron. When asked about romance in the film, Oscar Isaac divulged the following:
"Mostly with BB-8. There's a real warmth there, there's a give and take, and that's who has his back in the X-wing. So if there's any romance for Poe, it's with those two little balls."
Cue a deadpan quip from Harrison Ford: "Hasn't it always been like that?"