I think the idea of having the summons do various things is a good one too, you just need to flush the ideas out more. The idea of DoT's is good but sadly status effects don't seem to do much in raids/dungeons so I would stay away from those. Maybe each summon does its own debuff with attacks? This would make a multi-SUM party have to figure out which order to do their summons in. So for instance Ifrit does really good single target damage and lowers the enemies armor due to heat, Titan does AoE and lowers their earth resist, Shiva does targeted AoE like the Freeze spell, and Garuda could speed up allies attack/caste speed? These are just what jumps out at me to make sure all the summons would have a place and still be at least a bit unique.