I've been thinking what if they're not breaking any supposed patterns what if they didn't give Ash a grass-type in XY or the regional bird in SM because the writers just didn't like them? Maybe they didn't feel they were the sort of Grass or Bird Ash gets.
Toucannon is overall very different from Ash's previous birds (who have more 'bird of prey' traits). If I was't told it was the regional bird I'd assume it was just the Chatot of SM. Also having a Grass/Flying Starter would make giving him another bird perhaps redundant. Decidueye just looks more like a bird he'd catch. Especially after Talonflame.
Same for Grass-all of his Grass Types by that point had been reptiles and starters. When I compare this trend with his Fire Types and Water Types I can also see they have connection between them as well.
What I mean is I can literally pair Ash's Water Types up: Reptiles (Squirtle, Lapras, Totodile), Crustaceans (Kingler, Corphish), Mustelids (Buizel, Oshawott), and Frogs (Palpitoad, Greninja). Seriously, how far a head do the anime staff know what's going on in the games? When it gets to the point what he catches hints to the next water type... Water Bugs (which in Pokemon include arachnids among other creepy crawlies) appear to be next. Of course maybe they'll skip it.
Fire Types can also be paired up: Chinese Novel Characters (Fire/Fighting: Infernape, Pignite), Mythical Creatures (Fire/Flying: Charizard-Dragon, Talonflame-Phoenix), and Generic Animals (Pure Fire: Quilava, Torkoal).
Point is while I like them a goat and a tree ghost don't fit with that we've seen Ash catch. I don't want to call this a pattern, but maybe a 'theme' or there are qualities that they're looking for in each Type that makes it work for Ash.