Wouldn't it be safe to assume that Rowlet's final evolution would have a far higher chance of being grass/flying than grass/steel or grass/ground?
It's safe to assume yes, but going by the whole 'Prehistoric influence' that grass starters have, I'm going to stick with my theory that it will go the Ornimegalonyx route.(Which is highly likely) BUT becoming Grass/Steel type. It can still go the Ornimegalonyx route and be Grass/Flying, but it wouldn't make as much sense due to the Ornimegalonyx being nearly flightless.(Although Archeops/Archen prove otherwise.)