Yoshi already stated in an interview a while ago that they are planning/working on your Magitek Armor mount will be the next companion to be released (he didn't mention a timeframe or patch target for it though).

Beyond that, he mentioned the possibility for a Wyvern companion for Dragoons (mostly due to all the FF XI fans demanding it & Yoshi not wanting to completely rebalance DRG due to making it a pet).

Past that, it kinda seems like companion status is the direction that Retainers are going right now (being able to equip gear & gain class levels).

I'd love for other other mounts to get the companion treatment!

Although as for more new ideas, I'd personally like to be able to do a quest(line) to get a Beastman companion from each Beast Tribe I've gotten to Trusted with (with a different story/line for each tribe of course).