I really doubt they'd change Rowlet's current dual typing. Thinking they'd make it into a burrowing owl is a bit of stretch. Idk I just feel like if they were going to give it a ground typing later they wouldn't have bothered with the secondary flying type at first. (EDIT: Not to mention I don't think they'd give the grass starter a typing that would be effective against fire, which kind of breaks the starter weakness/effectiveness triangle.) So if any of them is getting a ground type I would bet it'd be Litten.
Water + fighting for Popplio doesn't sit right with me. Sea lion flippers aren't really conducive to physical attacks, unless it's just slapping people? Popplio to me has greater potential for speed and special attack. I was hoping for water + psychic (manipulating bubbles and balls and balancing and stuff) or water + fairy either of which could have made for some majestic final evolution designs.
but I guess we're getting some kind of weird butler sea lion
A burrowing owl is an interesting concept, and I can understand the desire to make the final evolution noticeably different from how it started out, but if this holds true then I'd honestly prefer if Rowlet stayed Grass/Flying instead of becoming Grass/Ground. Rowlet is still my favorite of the bunch but I'm a bit less excited than I was.
Also not too excited about Popplio potentially becoming Water/Fighting in its final evolution, as other potential dual-typings listed here would be more interesting. Pretty much the only neat thing about it to me is that now Grass, Fire, and Water will all have had one starter's final evolution with the Fighting secondary typing.
Litten staying Fire is fine by me, albeit a bit bland considering the other two have dual-typing. It will be the only Fire type starter to stay strictly Fire type through all its evolutionary forms since Cyndaquil/Typhlosion.