I'd like to see more items that enhance the eyes and mouth, drawing attention to the face without necessarily changing the expression or the eyes themselves. For example, we have some make-up and tongues for dogs but not a lot and I'd like the shadowy eyes/eyelashes more if they didn't look so drastic or out of place on most pets. I do enjoy changing the expression sometimes too but there just aren't that many neutral options. Even the make-up is mostly very stylized. I'm not trying to make all of my pets showy, dressy, or even feminine; I'm just trying to emphasize something I love about the pet! Maybe some very subtle makeup/eyelashes/eyebrows, different colored fur patches around the eyes or mouth, less drastic eyebrows, and I guess freckles could be added for a more natural touch?
I do enjoy having contacts if I want to change the eyes entirely, but sometimes I want to give focus to an otherwise busy composition or to add impact to an otherwise very subdued/similar color scheme (not as a complaint just as an enhancement.) I want to guide the viewer to my pet's face because that's the most obvious focal point. I would also gladly take more contacts as well of course. For all species! In particular, the few cat eye contacts that I have are kind of wonky. Some of the dog ones are too... I love how the bunny eyes can be used for horses, rats, and a few other pets but I also wish we had some specifically for all other species.
I'd also like to see more wigs without hair covering up most of the face. Sometimes I enjoy that, but sometimes I just want to frame their face and either the hair blocks the eyes, some of the markings, or in the case of some older wigs, there's a random stray hair that just looks awkward and the placement drives me nuts. It'd also be nice to have more short hairstyles to off-set all of the longer flowing locks. I know a lot of the best ones are for CS dollars but even a lot of those sort of follow this trend. Maybe having some more generic halo effects or something more natural to achieve that could also work? I would also love to have more wigs in general! And more facial hair... my problem is sometimes I can't get the facial hair to fit right or look good on the pet or match a wig.
I also wish there were fake noses, for lack of a better term! It's not often but sometimes I want to change my dog's nose, and I think other pets could benefit from that too. Just some basic colors would be great but also a few fantasy colors would be nice as well.
I recently started making fake eyes and noses on a few different species, with varying success, using stuff like the sugar pearls, coal, and chocolate beans and adding new highlights with the little magical motes. I think offering a single "build your own" set could be really interesting but I realize that'd be even more art for the same result.
I also think having more fur patches for the body could be interesting, either random and natural or more like tattoos? (I love using that one bunny tattoo we got a while ago.) Sometimes I love a pet's color scheme but I hate the pattern on them and my only option is to cover them with as much clothes as I can which doesn't always work out. Although if I do love their patterns, then I can't put any clothes on them or it'll cover most of it. So, it'd also be nice to have more collars or jewelry and maybe swimwear or like those flowing goddess scarves. I'd be happy with small crop tops, tank tops, and sleeveless tops and having separate sleeves for the cold shoulder or just a custom look could be cool. Maybe very short shorts... IDK I'm not trying to make them sexy, just not sure what else would show the love. Of course even if I put nothing on them or a minimal amount of any items, I still love them. I guess that's not very helpful though.
It'd be cool to look at some of the most popular dogs with hair and if it's possible, maybe turn some of those into wigs or use as inspiration for wigs. Same for their outfits! (And that applies to other species too.) Or go back to some of the older wigs and make new versions of them with new hair colors, again if it's possible. I realize some of those may be files lost to time but again just using them as inspo would be nice too. Sometimes I want to use a certain wig, it looks good, but the outline is too harsh or it doesn't come in the color that I want.
Sorry for the long ramble! I hope some of that made sense. I also realize I'm asking a lot so if even one of these ideas sounds plausible, that'd be amazing. And sorry if I accidentally repeated anything someone else already said.

I love the advent calendar costume for horses and I'd love to see more designs like it!
I'd also appreciate some that don't have the blinders covering the eyes. I know I sound so picky but I enjoy having the option to choose since sometimes I absolutely love the eyes and don't care how much of the coat is covered and other times I hate the eyes but do care how much of the coat is showing (well, hate is too strong... sometimes the eyes don't feel right or I can't tell if they're even colored in since horses have such small eyes to begin with and mine aren't that good even when I'm wearing my glasses lol. But more often I like the eyes and the coat has spots that triggers mild trypophobia, for example.) Pretty much for every pet those are my main considerations btw.
Overall I loved these and need to hunt down more of my horses that I'm happy to place these on lol.