U]Demi-Ifrit[/U]
The first Demi you'd get at 30. Basic attack will be the fastest out of all of the demi's. Main attack would be the Egi's Aoe damage, ifrit is all about raw strength so he should be offensive. Ifrits support could summon one of his nails where he stands, it gives off an aura that lowers physical resistance to enemies or increases damage to Allie's. This enables pet positioning. Enkindle would be the same as Egi's but with a stronger DoT and possibly higher damage.
Demi-Garuda
Range caster with basic attack being fairly on the weaker side.
Main attack: ranged AoE Dot
Support: AoE haste but with a small radius. (Reduce global CD)
Ult: Line AoE damage. (Like a mini ranged attacker LB)
Demi-Titan
Melee heavy hitter low basic attack speed but high damage
Main attack: cone attack that leaves an attack down debuff for a short time
Support: I was wanting a Cover effect for Titan, he becomes tethered to an ally and he absorbs all damage delt to the ally.
Ult: Titan Egi's ult but with a Slow effect on top of it.
Demi-Bahamut
Ranged attacker, heavy damage but low attack speed
Main attack: heavy single target attack
Special: remove the casting time for Ruin III (since DWT is changed as a timer for your demi's, this effect wouldn't be a thing so I thought we can instead have the effect as an ability)
Ult: his normal Ult is perfect, giant AoE damage.

This would be how I'd rework the summoner overall, to have a system that would feel like classic summoner of summon a dude to have a powerful effect and he bolts. Lemme know some of your thoughts of this hypothetical rework