I was thinking it would be interesting if summoners were allowed to summon primals, but you could only choose one of them. This also would not violate the lore and I think it would make for interesting gameplay. By forcing the player to choose which summon they want to make a pact with, it makes each summoner unique. You'd end up with some people who have ifrit, some who have titan, some garuda, etc etc. Each would have their own pros and cons. For example ifrit may be more suited for battle where as something like garuda may be more support oriented. So by choosing a certain primal it determines the summoners role in a party.

Of course summoners would also have the basic abilities from the classes like cure so forth. So depending on what primal you choose and how you fill your action bar, you could have a pretty unique class.