The summons that SMN will have access to at first are Ifrit, Garuda, Titan, and Leviathan; yet Ramuh's artwork was shown and not Shiva. Good King Moogle Mog XII doesn't count, but I believe that he will be a boss for us since we can still obtain Primal Weapons.

I had a theory that as an Arcanist, if you fight a Primal and defeat it, you obtain the gemstone of the fallen Primal. You may be later able to equip it and then summon its essence. That begs to differ if you can only equip/summon one at a time, though.