If they added summer and beach stuff, I think it'd be funny if they added a speedo for guys. Can you imagine how oddly amusing it would be to battle some random player in a speedo and stuff? But, honestly, the best things they could add to customization IMO is more eye colors, odd eye possibility, more hair colors, highlights, more haircuts, and definitely more clothes to wear. Maybe even some version exclusive outfits? Like OR has a groudon armor themed outfit and AS has a kyogre swimsuit outfit.
Wow I just realized how my amount of clothes tripled since I got X and Y lol