This is one of the few things that has been bothering me, is the fact that the companion system is being underutilized what with the Chocobo being the only companion thus far. So I wonder: what or who do you think should be used with the companion system here's mine.
Ark (hear me out)
Item to Summon: Lapis Lazuli (I know you need plumes to summon him in 9 but I have no idea as to who or where we would get plumes from)
Companion mode: After the cast time there will be a quick version of Arks summoning animation where he follows and fights as his Mech Mode
Mount Mode: This would see Ark turn into is Ship Mode where the player will then jump to board him and then fly.
While Ark (being Ark) doesn't have much in the way of Customization I'm sure he will be viable. Now it's your turn, who do you think would work with the Companion system?