I'm assuming you mean 'associate summons with jobs as a mental exercise' rather than introducing this into gameplay proper, which would not be a good idea.

Paladin: Knights of the Round (obvious knightly themes and all)
Warrior: Brothers (they use axes, two brothers are a major part of the WAR questline... why not?)
Dark Knight: Odin (has traditionally been associated with darkness and the Dark Knight job since FFIV)
White Mage: Alexander (Holy elemental summon)
Astrologian: Eden (space/heaven imagery association)
Dragoon: since Bahamut's taken, Midgardsormr or Mist Dragon.
Monk: Titan or Maduin (probably the summons that rely on their fists the most)
Ninja: This is the hardest one for me, but probably Phantom from FFVI (makes everyone invisible).
Samurai: Yojimbo (easily the most Samurai-themed summon in the series)
Bard: Siren (Do I need to say why?)
Machinist: Ark (of course they should be able to summon a transforming mecha)
Black Mage: Kujata/Kjata (uses fire, lightning, and ice) or Shinryu (again, multiple elements)
Red Mage: Phoenix (because rez mage)