The one time they got asked about this was some time back during HW's cycle; it was asked about adding other companions like a dragonette. It was their usual answer of "we're not even thinking about it" with a side-explanation about how they'd have to animate the new companions (to be different from whatever's already there, for some reason) and add a new and separate leveling system (don't ask, they hardly elaborated on it). I couldn't find this exact interview even if you asked me, I'm not an encyclopedia of every interview they've given.

It seems like they are perfectly content to leave chocobos as the only companion; it having fulfilled its purpose as a crutch for what little open-world content there is, and seeing no point of further attention than a ostricken to paint and dress-up. After seeing Squadrons and Egi glamours, this is hardly surprising.