I don't think we'd end up being able to summon the primals themselves, since, you know, the whole tempering thing, and the trying to destroy them permanently thing, and the massive drain on aether thing... doesn't really tie with the story very well. But you never know, they'll probably find a way to do it.

I don't want to get too carried away with theories, but I'd say it'd be more likely to summon some lesser 'primal' or maybe one of the twelve - even then it'll probably be story-related (i.e. one-off). Or maybe a new class specific SMN LB?