I'm trying to think of Hidden Abilities for these guys. The Kalos starters got HAs that corresponded to their theme, with Chesnaught getting an ability that negates certain attacks similar to how a soldier/Fighter class can glance certain blows, Delphox got Magician because its a mage, and Greninja got Protean to reinforce the sneaky ninja motif.
Safe to assume these starters will get brand new HAs as well, and if the circus theme theory turns out to be correct, it could open up some interesting ideas.
I.E. Rowlet seems to be based on a circus maestro/ringleader, one that controls how the circus runs with its acts. So, maybe it gets an Ability that serves as a permanent Torment, to prevent the opponent from using the moves it wants to use at its discretion, because Rowlet runs this show here. That could be super useful against mono-attack sets, or certain coverage sets if Rowlet's packing Protect.
Pipplio is a circus seal, evidently. Circus seals perform grand tricks and get rewarded for them with fish on scene. Maybe an ability that rewards Pipplio for nailing critical hits? Maybe with something like a stat buff, or to go with the fish idea, a recovery boost? It would learn moves that come with high-crit rates to make a viable set with the ability - maybe even get a water-type Frost Breath?
Finally, Litten. If going with the circus theme, then Litten is a lion/tiger used for taming tricks. Unlike the circus seals, the felines are often viewed as being more oppressed on stage due to the use of whips to control them....maybe an inverse of Pipplio's ability? It takes a critical hit, it gets an array of stat boosts to get it more focused and battle-ready? And at the same time, because the whip cracks are common, the felines would be used to them, so maybe Litten's ability lets it cut critical hit damage in half, so that the damage doesn't offset the stat boosts. Flash Fire and Water Absorb and Lightningrod completely negate certain attacks, so I don't think turning the boon of critical hits around on its head is a strange idea.
I dunno, I'm really interested to see what they get for HAs. I'd like to be close with my guesses.