Fenrir in a duneon? Really SE, reallllllllly?

[ZReport]

Just because they put these summons in as dungeon bosses, doesn't mean they can't reuse them elsewhere. Before Siren was placed in Pharos Sirius, she was made into a main story quest fight, one that was an allusion to the Siren fight in Final Fantasy V (in the Ship Graveyard). Nothing is stopping them from re-using these summons in another manner in the future. For reference, here's a list of past Final Fantasy summons which exist in Final Fantasy XIV so far:

Bahamut
Carbuncle (Arcanist Summon)
Odin
Ifrit
Shiva
Titan
Ramuh
Garuda
Leviathan
Kujata (Marauder Quest Boss)
Atomos (1.0 Boss; Labyrinth of the Ancients Mid-Dungeon Boss)
Catoblepas/Shoat (Mid-Dungeon Boss)
Siren (Main Quest Boss; Dungeon Boss)
Diabolos (Dungeon Boss)
Fenrir (Dungeon Boss)
Zona Seeker (S Rank Mark)
Midgardsormr (Dead Carcass Wrapped Around Imperial Airship in Mor Dhona; His Death Allows Current Primals to be Summoned in the First Place)
Belias (Summoner Trial Boss)
Gilgamesh
Zodiark (Ascian's Overlord in Story)

Not in Game Yet:

Cait Sith
Typhon

For others, emphasis has been shifted away from them being all powerful beings into beings which populate Eorzea as a race or group (Tonberries, Sylphs, Chocobos, Moogles). As people can see, they've already added a good deal of summons and re-purposed them to fit the story in some way.