There are so many cool ways to do this! There’s the FF11 method or the pending mmo Peria Chronicles method. Both valid and engaging. What I really want to see though is an achievement based summoner system. I’d like a system that rewards hard work. Maybe a quest string that unlocks a summon? Or since this is a sandbox, maybe a discovery system to unlock summons? Maybe, go to the top of this mountain and ‘trap’ this guy to unlock him as a summon? Find a random world spawn?
I also think an existing summon should drain your mana/health/whatever. Maybe the stronger the summon , the greater the drain?
Afterthought: maybe a pixie summon type system? Buffs/heals/debuffs? It really depends on if we went the summon to be the featured character or our own character to be what is featured.