The principal issue with summoner is target swap, I think that make a spell to erase Bio, Bio II and Miasma from your current target and then apply the erased effects with his duration in another target could be useful

For more flavor: It could be fun if they remove fester and give at every summon an useful ability that costs aetherflow charges, it could be some like a DOT for Garuda, a direct attack for Ifrit and a defensive ability that affects the SMN for Titan.
An ability that let you swap summons without wipe 1/3 of your mana could be nice too