I was thinking that possibly the characters start out with only one summon, obtain a second pretty soon after, and then the rest they have to work for throughout the rp. They would also have to train them to make them stronger (excluding the first one, although I did have an idea for that). I think both of these would help contribute to the character development quite a bit. Another idea I had was that at first, the first summon can't metamorphise (That would be the correct term :v fuck you pokemon for not using the correct term lel) unless in dire need, such as the character being in danger. However, as the rp goes on both the character and the summon learn to control it and use it on whim.