I like myrtle (vinca minor) -- it's a pretty nondemanding groundcover, assuming that's what you're asking for. Otherwise, we probably need some facts about where you live!

Beautiful sidewalk! I have been having good luck in the mid-Atlantic area with Knockout roses and salvia (some white, some blue). In just two years, they have filled in nicely. I have an edge of dwarf boxwood that give me something in the winter.

Hard to make meaningful comments on plant suggestions when we have no idea where on the planet you might reside :-) A geographical region or the nearest large city, if you please.

Just an FYI but Vinca minor is really best suited to a shadier position. It can scorch and the foliage will bleach out if in too much sun and growth will be stunted.

Hi everyone! Thank you, I live in Boston. I was thinking a dwarf tree??

maybe mulch and day lilies?

or mulch and low growing grasses or annuals?

how about some pebbles/rocks with a few plants?

Some option with shrubs

Which direction does the house face? That's the next question after where in the world you are.

With the exception of small Japanese maples, most small trees aren't all that small. So they aren't going to fit in that space. Tree form shrubs are an option, but I haven't seen them around in a while. I can see a couple of blue rug junipers there, or a dwarf cryptomeria, but I'd think about moving the peony if you want something human sized in that space.

BTW, ignore the flat-earther. If that slope hasn't moved so far, it isn't going to suddenly erode. Also, I'd think about putting railings on that porch. It may not require them by code, but they can save your dignity, at least, after an ice storm.

Chamaecyparis obtusa 'Spirited' as the evergreen focal point. Fill with more peony (2) and the creeping thyme groundcover you already have.