One key thing some people hate is that Demi-Bahamut is tethered to Aetherflow for max DPS output, while another is that he follows the player around instead of staying put like Garuda would. These two issues are tied to the way Wyrmwave works; basically it's not an auto attack. Personally I would love to see action create procs instead. Here is my Demi-B changes I posted in another thread.

Demi-Bahamut:
*Make Wyrmwave his auto attack, change to 3s recast.
*Ruin, Bio, Miasma have a __% chance to change the next Wyrmwave to Mega-Flare - 250 potency
*Change Enkindle Bahamut recast to 45s or once per summon.
*Change Ahk Morn with Tera-Flare: 1360 Potency attack (combine two Ahk Morn into one big attack)
*Add party wide Buff, Eikon's Favor - Critical Hit rate increased by __%.

As for Devotion, just make it a party buff or AoE buff where players have to stay in range of the pet.