So far, looking at more tamashii lines than just this one, I'm not counting at all on any kind of additional figures packaged with anyone. Even goten and trunks isn't a guaranteed 2-pack. I think this is because they'd simply rather sell two things more cheaply, and cumulatively probably even make more on it, than offer more expensive combined packages.
So continue fantasising to your delight but I think it's never happening. I genuinely don't expect puar or chaotzu with yamcha/tien respectively anymore either. I don't think we'll ever get side characters like you mentioned, if even just because there's too many stuff that still has to get here and we don't get many releases per year. The line is too prestigious for it, commercially they'd rather invest that time into a more clearly sellable character.
I think it's best to simply face that this line is for main characters, and if by some miracle we do get a wider scala, take that as a lucky break (17&18 in that range already). If we get anything in multipack form it's gonna be something like unarticulated differently posed Cell Jr's., probably in SHZero.
I think we won't see Bulma unless they start a dragonball line. (Which I'd love.) I don't see us getting a character like King Kai at all. Tamashii seems to be very focussed on iconic scenes from the anime, which is commercially very sensible. So if you wanna know if a character has a chance to get made, I'm inclined to say ask yourself if you can think of a very iconic scene from the anime with them in it. If the answer is yes, I'd say that's the best chance-boost a side character can get.