I want that Rowlet bad! Though I always tend to go for the Pokemon that seems to be least loved so maybe I'll go with Popplio.
Really looking forward to their secondary typings as well. Grass flying might hold up but would be cool if it changed into something else. A burrowing owl thats Grass/Ground would be pretty cool. Litten's Japanese name and coloring give the idea of molten rock so I would love if we got a Fire/Rock or Fire/Ground out of it. I don't know what Popplio could be, maybe it's clown theme could lend it's self to Fairy?
None of those typings are unique but with Tropious and Shayman as our only Grass/Flying Pokemon, Camrupt/Primal Groudon as Fire/Ground, Magcargo as Fire/Rock and Azumarril as Water/Fairy I would be pretty content with any of those secondary typings. Doesn't matter if they have quadruple weaknesses, they can join the 4x club with Charizard, Swampert and Torterra.
With the circus theme going we've got the clown, the ring master and I guess just the performing tiger. Not sure how those things could play into their potential typings. When I think of clowns I guess I think of fairy because of Mr Mime. With the others who knows?