There are no pre existing characters interesting enough to warrant them becoming a partner. Even a character like Jeri felt more significant than Hiro and Ruli's friends who I frequently forget exist.
Personally speaking however, I'd rather we just not have an extra member unless they serve and antagonistic role against our group for the most part. Like if Dracumon actually found a Jerkass of a kid or something and they stayed on opposing sides. Otherwise the concept doesn't interest me especially when I more want to get into the heads of our current members and I'd rather not have to share time with some potential extra who may or may not be worth the screen space.