Any healer Job will be viable provided it has the following tools:
A) A spamable, potent single target heal (to use on tanks).
B) A spamable, decent AOE heal.
C) A spamable, decent single target heal (for spot healing).
D) The MP to use these consistently for the fight.
E) A Raise ability.
All the three healers currently in the game have these abilities as their base toolkit. So:
A) Healer/debuffer.
B) Healer that stacks points/tokens. Like NIN combinations but for healing.
C) Healer with consistent, but weaker, buffs than AST, with each retaining some unique buffs.
D) TP based healer with weaker heals that can be sustained, can use MP for burst healing with a TP skills to manage MP (like PLD), probably a melee based healer. (RDM/PLD-ish)
E) "Blood Mage" healer that uses their own Health pool + drain life.
F) Sky's the limit - there are more possibilities.
FF Jobs:
Dancer(C/D), Chemist(B/C/D), Cleric(C), Druid(A), Shaman(E), Blue Mage(A/C/D/E)
(Could explain, but char limit...)