Can I just say that I love the new trainer models and even their base outfits? Hope the hats are available for both genders. AND THE GIRL HAS A BACKPACK. Oh finally!!!! I hate traversing the region with a purse XD. Seriously, I could do nothing but love her when I saw that. It makes me wonder if more options will be cross gendered, like shoes, hats and bags being the same for both? Perhaps even haircuts???
Personally, I think it's safe to assume Customization is back and will be better. Real question is by how much? I'm glad to be out of a tropical region, SUMO had way more clothes than XY but not a lot of styles, so it felt boring. We did get more hair cuts though, more hair colours, eyewear options, removable hats, jewelry...
My safe prediction is that all the basics from SUMO return, we will have more hairstyle options, boys might even get bangs or the hair options will be the same for both, and even more hair colors to include ones like green, pink, etc. Some options won't be available until post game. We might get an additional layering option like "jacket", they'll be even more clothing available than SUMO and these will actually feel like way more because there will be way more styles to choose from besides tropical wear.
I really think we are going to se a merging of certain base clothing for the genders (hair/hair cuts, hats, bags, shoes) just because it both simplifies the work for them but also makes it feel like they did more.