Hm off the top of my head, make Spirits a Support like ability. Let's say the Summoner can summon a spirit and synchronize it with a player, with that Synchronization in effect they gain access to a list of spells that they can only use on that player to enhance them.

Example:
Fire Spirit
Infused Stat Boost:
STR+10
ATT+50
Crit Hit Bonus +5
Fire Resist +50
Water Resist -100
Spirit Stress 0/50
Base Perp Cost: 15/tick
"Spirit Stress based on summoner's skill the average tic is 1 every 3 mins with lower skill it's more but once the limit is reached the user receives "Spirit Fatigue" and is unable to synchronize with a spirit for a set amount of time. The effect also doesn't wear off if you die or change jobs."


--Spells you can cast on the target--
Fire Rage "Increase STR" (.5 Stress Based increasing by .1 every cast) EX. 2 cast would add 1.1 Stress MP cost: 20 Perp. +1
Flame Shield "Increase Fire Resist" (.5 Stress with .1 Every cast) MP Cost: 20, Perp. +1
Fire Conjure "Add Flare to targets spell list" (10 Stress) MP Cost: 200, Perp. +10
Melt "Increase damage against Ice based foes" (5 Stress) MP Cost: 100, Perp +5
Fire Boost "Increase Fire Based dmg, including SC" (5 Stress) MP Cost: 75 Perp +3

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It's just an idea that could add flavor to a SMN, while it would be impossible to give full control of a spirit because of BLM and others jobs that may find it unfair to be able to control the actual actions of the spirit. If one were to tailor spirits to act as a vessel to enhance other players it could work. By putting the proper limitations on it while it wouldn't become something to overshadow another job but an opportunity to create and flourish more strategies.

With the idea on hand while it would be possible to infuse spirits on 5 players at once one will have to take into account the perp cost.
By casting 5 spirits you would be at 75/tick of MP loss if you consider the idea of trying to focus 5 elements on 5 different targets, then on top of that trying to enhance them would add even more stress to the user. Basically I was trying to think of something that would sound like something a Summoner would have the ability to do, and the idea of them being able to enhance other players through spirits does sound like something they should be able to do lol.

--Hm the perp cost is prollya tad costly based on the actual amount of MP a user has access to so maybe the base perp can be 5 and slowly increasing based on the spells the user adds to the target lol.