Honestly, since I've been dressing up all my gift pets, here are a bunch of item suggestions I'd love to see:
Pet beds, not just ones that are sized for the pets, but some with static sizes to use as background props.
Elemental wings : wings that look like they are made of fire, ice, water, etc. Or just wings with elemental effects such as flaming bat wings.
Manes for lions! Love to change lion main colors are add manes to lions who don't have them.
baby/toddler items like sippy cups, play phones, onesies, backgrounds that look like nurseries or have kids drawings on the walls as well as kids drawing poster type items we can use to decorate backgrounds.
More house/home/dwelling backgrounds, we have very few items that just look like a pet would be inside their home somewhere, especially would love to see some kitchen settings.
50's set with iconic hair styles, clothes, and settings like car hops, diners, drive in movies, etc.
More hobby themed items: crochet, knitting, beading, stain glass crafting, ceramics, etc, including backgrounds that would be useful as art studios, maybe one with a kiln, or shelves that we can place items on.
Casual hoodies with hood up/down options. Hoodies are so popular and we don't have many up hood options that aren't costume based.
More scene/goth/emo clothing and hairstyles. I know I'm kinda biased on this one, but I'm always going to be a 2000's emo/goth kid, so always love to see more representation.
SHOES SHOES SHOES!!! It's so hard to finish outfits because we have so few shoe items, especially casual shoes styled after say, Vans or Converse.
And in general, more items that fit less common species like rabbits, scorpions, hamsters, lions, deer, etc. These pets can be really hard to dress up because so few things actually fit them.
I would also love to see items that just add patterns to pets, like tiger stripes, leopard spots, polk-a-dots, brindle patterns, etc.
More perspective decor for making scenes, kind of like how the RPG event backgrounds had separate items for the rocks, wisteria, etc that could go in front. I'd love to see small trees or forests to put behind pets, groups of flying birds (You all know the ol' V shaped bird flocks in backgrounds), mountains, hills, etc. Just things to give us the ability to build on top of existing backgrounds) Also more night time items, we have very few items that look like they are in the dark, kind of like this tree from the summer event:

, so it can be really hard to customize night time backgrounds. On this note, similar to the picture frame glass panes, I'd love to see items that allow us to cast a hue over an entire scene, so that we can make things have a certain feel, such as overcast, or give the whole scene a red or teal hue.
Sorry for this long list and I apologize if I'm asking a lot here, I just love dressing my pets and there are so many things I'd really love to be able to do.