Summoner having a pet(s) is quite traditional approach (and there is nothing wrong with that). However, maybe there could also have a chance to call a temporary creature, which is a more like a spell rather than pet... Meaning, maybe you can for exanple summon a dragon, which flies one or few times over the battleground and burns the targeted area. Or maybe some of the summons will posses the summoner and player can that way control the summoned creature. There can be variations between permanent and temporary posses.