I saw someone else do it, so here's suggestions from myself!!
Side desk, like to put next to the couch
coffee table (maybe make it so we can set things on top, like if we had a small flower pot)
maybe a chair the cats can sit on at the desk?
rotate items so they face where we want
kitties get angry if you give them too much attention
rare kitties that are based on made up entities (vampires, angels, zombies, medusa, lit anything you like)
human kitchen stuff for kitties to jump around and play on (since there is tile, i thought itd be nice)
litter boxes and if you wanna add the mechanic of cleaning trash can !!
straws as an option for toys bc for some reason cats LOVE straws !!
personally, i believe it should be easier to get coins or things should be lower in price.
mini games !! instead of lowering the price, you can add mini games!! a lot of cat-game creators think like oh put those basic minigames like the switch blocks -- which is fine -- but it'd be a cool mechanic to be able to clean the kitties and do other things to take care of them and maybe a minigame
close up of playing or petting?
following the last one, a bathtub, and possibly if you like other bathroomy things !!
walls !! only issue w/ walls is that the kitties do get stuck on random things a lot like food bowls n stuff that is solid
the ability to add/remove/move plants like trees, and maybe there can be a mechanic to plant trees and flowers and over time of playing the game they grow?
insects would be really fun, like if there were flies flying around and moths attracted to lamps, and maybe kitties can chase them too. itd also be cool if there was a chance for a kitty to find an insect and make a bubble with a (!) and if you click it fast enough you get a little scene and you can capture a bug in a jar (kinda like fishing in stardew valley)
last one would be adding onto the other persons suggestion of a shelf!! we could put jars on shelves (with sticks and stuff inside, like theres automatically a terrarium to fit the kind of bug inside and its little pixels of colour far away
cool bugs to capture suggestion : any kind of mantis (pref orchid mantis imo :3), stick bugs, fire flies, beetles, moths, butterflies, dragon flies, lady beetles, crickets, bees, CENTIPEDES AND MILIPEDES !!!!!! (my fav besides mantis'), ect
also, you could add an option to disable bugs.
insects have a possibility to put multiple in one jar based on the size of the jar and insect (like once you catch one it has an option of what jar to add it to, so like you buy the jars and then decide what one to put them into, and ofc there should be bigger jars and maybe you could add a tank for many bugs, fish, snakes, ect that the cats would interact with.)
auto feed for wet foods
maybe make walls more visible and items to mount on the wall
kitty playgrounds or outside things to jump on
sandbox and maybe there can be a brush to clean up messes kitties make if they track stuff out of the box or if they spill food or water or if (piggy backing off of other idea) they use the bathroom outside of the kitty box (and since that is a kinda gross mechanic you can add an disable option)
twister. i dont need to explain.
stupid silly idea : music box and once it plays music cats start dancing like parrots in minecraft. jk, but itd be cool to add an option to put on music.
Past kitties come back (specifically bc i was afk and missed some really funny kitties)
page you can click as a tutorial/faq of how to do things that either pops up when you start and you can click out of or theres a button to click into it if you are lost or forget somthing since many people get confused as i can see in the comments.
thats all i can think of rn !! i really really love this game and would love to watch it evolve <33