Skills- It could be interesting to "merge" two summons into a single one, more powerful, with differents effects.
The pool of HP would merge as well.
Also, there could be "special summons" (Obtainable through side quests), with special attributes and effects.
I've put these ideas into a rudely drawn scheme (with a few ideas on what fusions could be envisaged) :
About the evolution of skills : - It'd be cool if their visuals could evolve (let's say every 2 levels) to reflect the gain in power.
- Each skills could have two branches, a variation of the skill with a slightly different effect/AP cost/Mana cost.
- I guess it's trivial, but maybe we could be given the option to choose which level of a skill we want to use (mana management) ?
Here's another scheme :