That's a really good question.
For one, Kawano was originally created to simply be an NPC shopkeeper character. P4 never showed who the shopkeeper was to Souzai Daigaku, so I figured it was okay to create my own. When creating her, I ended up enjoying her as a character and decided later that I wanted her to be included as a social bond.
For original characters, I have a lot of plans for them. I don't want them to be resorted to shopkeeper characters, although Kanji and Yosuke WILL also act as shopkeepers. For the girls, namely Yukiko, Naoto and Rise, I want to make sure their storylines integrate well into the main game and they have more freedom.
I also don't want to rush their design and implementation.. I have done this in the past, and it makes it a lot harder to work on these characters after the fact, especially once players have already put affection points into each one.
Hopefully this a nswers your question, and to ease your mind more (possibly?) my focus for next girls to insert are most likely Naoto or Yukiko since they fit really well into where the story is going next.
Also, I just want to quickly say that yes Yukiko, Marie, Rise, Naoto, Aika, Saki, and Ai are all planned for this main Inaba concept. Some of them sooner appearing sooner then others to help with storyline. I will also say that Maya, Yukino and Lisa are planned. As well as Ann, Makoto, Haru and Futaba.
Many more are planned as well, but those main girls are a 100% going to be implemented into the game.