Well I may have found more information on that!
I can't confirm the pet 100% yet, but wowtcg.com is having a Throne of the Tides sneak peak event and tournament. They've listed the loot card prizes as follows:
5 Wins: 4 Throne of the Tides booster packs, 1 Wasteland Tallstrider Loot cardThe first loot card sounds like the Tallstrider mount that was rumored to be coming soon. The second loot card seems like the misc "fun" loot card. And last but not least, Bloat the Bubble Fish sounds like a fish in a bubble to me. If this isn't an indicator of what the next TCG companion will be, I'm not sure what is! I'm hoping to see a datamined version of this pet once 4.3 hits the PTRs. :D
4 Wins: 3 Throne of the Tides booster packs, 1 Throwing Starfish Loot card
3 Wins: 2 Throne of the Tides booster packs, 1 Bloat the Bubble Fish Loot card
As exciting as this prospect is, keep in mind that things could change and we may not get a "fish in a bubble" pet afterall. Regardless, I'm still excited and anxious to find out more about the next TCG common loot card!