Make everyone else into catgirls too

Sophie: "A-hahahaa..."

You inspect your hefty, stirated arm and clench your subatomic-particle-sharp indestructible claws into an indominable fist. So much power, and just too much pleasure. All in the life of a catgirl, you think.

Sophie: "Wouldn't it be just wonderful if everyone could join in on this unspeakably delectable thrill? The whole of humanity... -no! All of creation, as one, pure, perfect beings - catgirls!"

A thought too tantalizing not to act upon. Some might call it reckless, but realistically? You have no one to answer for - in fact all of Reality now answers to you. When you snap your fingers and tell Reality to change every living thing into a catgirl, Reality says:

Reality: "Yes, Your Highness..."

In conclusion: Reality is the ultimate simp.

The change is very much instantaneous. All animals, insects, bacteria and viruses are transformed into a single gender of a single previously non-existant species: homo felis femina. Of course, due to the overwhelming disparity in the numbers of bacteria, it'd've been outright foolish to transform all specimens into the same size. No no - all animals will keep their original biomass. This leads to catgirls the size of single cells hunting, living and lounging about in the bodies of other, larger catgirls. On the other hand, massive, mammothine catgirls roam the savannahs of Africa, while even larger catgirls, previously whales and giant squid, inhabit the depths of the ocean.

While their physical size remains, all instances of homo felis femina are equally intelligent. This leads to some rather unnerving complications, such as that many types of decomposition and fermentation no longer occur, and that all larger animals will host tribes, and eventually entire civilizations of bacteria-sized catgirls in their bodies. To allow this, you've made homo felis femina an extremophile of the highest order: it can survive anything that any one previous lifeform on Earth could survive. Thus, instances will not succumb inside gastrointestinal tracts, in deep ocean or even in the vaccuum of space.

Sophie: "Lol, what a mess. Even with these precautions, it's gonna be a huge ecological clusterfuck..."

With a thought, you evaporate all of said complications. The world, despite consisting of a single species of animal, is now totally sustainable in the long term, and allows for comfortable living to all homo felis femina.

Sophie: "That's better! Now, what else could I do..."