A quick search revealed no topic about this since the announcement by Yoshi-p that Summoner will appear in the game.
For those of you who have not yet heard, summoner will not be summoning the prime avatars such as Ifrit. Those will be restricted to grand company summons - i.e. top linkshells per server will be able to summon them such as there is never more than one of any given avatar in the world at a time.
Summoner will be able to summon lesser spirits at will. There are a variety of these in the Final Fantasy lore. Here's a list of what I have so far:
Chocobo (everyone should be able to summon this)
Carbuncle
Sylph
Goblin
Bomb
Mindlayer
Cockatrice
Whyt
Golem
Catoblepas
Syldra
Siren
Unicorn
Maduin
Phantom
Midgardsormr
Ragnarok
Lakshmi
Crusader
Raiden
Cactuar
Gilgamesh
Kjata
Hades
Cait Sith
Moogle
Magic Pot
Tonberry
Quezacotl
Valefor
Yojimbo
Magus Sisters
Adrammelech
Mateus
Belius
Hashmal
Exodus
Famfrit
Salamander
Air, Dark, Earth, Fire, Water, Ice, Thunder, Light Spirits
There are more, but I think these would be reasonable. I don't expect them to add nearly this many. Adding a single one would be a great deal of work. I just hope that the developers remember some of these great summons for the future.