Thinking more on this, I do like the idea of summons/primals taking form as in older FFs, not XI's "Always Out" Method. Big MP cost, big damage. Add a timer for recharge, give them smaller summons or skills they can use to supplement their main Primals. Having to fight them before earning them (bigger primals only), should also be included.

Having non elemental Summons with unique skills (Oh how I overused Sylph and Asura in FFIV.) would be a welcome addition. Not all summons need be offensive either. There could be an Absorb primal capable of laying on 3+ debuffs at once.