I liked the game even though there is only one fusion, I feel that you can put more characters besides the two girls.
I'm sure that you will put another one with different points, even with a good detail.
If you want an idea for a future game, I will give you this as a way to give my support to your work.
Create a Pokemon variant where they create fights, only instead of catching creatures, you have to create friendships with other friends where you can create or fuse each girl to have a strong team.
Depending on the selection or capturing a friend to be able to face another rival, something like if you want to fight, you would have to fuse with the appropriate girl of any element that they can make compatible to fight by your side or something like that.