You could have a nice rock garden above the rocks and boulders. I would love to see Creeping Phlox tumbling over those rocks. Creeping Thyme works well too. I like recommending Fragaria virginiana (Wild Strawberries) as ground cover. For your northern location, Arctostaphylos uva-ursi would look nice too, and you can't beat Prickly Pear for summer POP of color:
See what you think of my list: https://www.helpfulgardener.com/forum/viewtopic.php?p=79896#79896
As for the tree, you probably don't want anything that grows really tall, so I'd stick with shorter trees. Crabapple sound like a good idea, or you might just stick with shrubs since a tree would completely obscure the house. I guess it depends on what you want. Personally, I think a tree would look better on the OTHER side of the walkway leading up to the steps, but it might just be the angle of the photos you're showing us.
My vision would be: Ground covers and low shrubs up the slope, and either a patch of lawn or a lush flower garden (I'm thinking Butterfly Garden) in the level circle where you indicated you want the tree.
Tall shrubs -- fragrant and flowering/fruit bearing -- that would grow as high as just below the top of the porch railings all along the porch: Native viburnums, rhododendrons, kalmias, ilex/hollies, dogwoods, hydrangeas... maybe lilacs.
If the downspout at the other corner of the house helps to water the area, you could have more moisture loving plants on that side of the steps -- summer sweet, spicebush. I'd LOVE to see a Winterberry holly somewhere to play against the red door and the white house in the winter time, but it needs a really moist area.