*Firmly sitting on my hands so I don't accidentally volunteer for something again.*
Just a few general things, mostly to help readers who haven't written banters but might want to give it a try.
First, mostly pointed at the modders, how many banters do you want for each NPC. Make it as specific as possible. If you want 4 for Aerie, 2 for Minsc, and 8 for Jaheira (although heaven knows why you would want that many for anyone), make a list. How many are already written.
For anyone else: think about the overall relationship between Cailean and the NPC. What would they have to talk about? How much of this material has been abundantly covered by other mods? For example, Valygar's distrust of magic is covered at great length. You don't need to go there again. What else might he and Cailean talk about? Maybe they both share a distrust of horses (so what if it isn't in the game already).
Take the idea and get an opening line. "Wow, Valygar, did you see the size of that hoofprint? I'd hate to meet the monster attached to that!"
The conversation evolves from there. Sometimes, it evolves in a way you never thought it would when you sat down to write. With the utterly lame first line I gave above, it might stay more or less linear and end with something like Valygar remarking that he never had to shovel up after his feet, or it might take off in a completely different direction, after following a couple tangential connections. Conversation is like that.
Banters don't have to be essential to the story of either NPC to be entertaining. In fact, as this is probably covered elsewhere, it's probably best if they're not.
I'd volunteer a few, but I'm already approaching over-commitment, and I haven't read enough about Cailean to say anything about his personality. Which makes me feel like a jerk.