While your house facade is indeed very symmetrical, its placement with the slope is not. And the large, red leafed tree (assume a Japanese maple) to the left side will negate any symmetricality you might want to achieve as well. So to answer that question, I doubt you need to focus on that aspect of design and work on just achieving a balance with the plantings.

And since you do not indicate where you are located, it is difficult to make any specific plant suggestions.

Would it be feasible to do some sort of retaining wall? I like how it brings up the height there, since your house is taller on that side ... and how it breaks up the long slope a bit.

I would be symmetrical with the plants that extend architectural features, but then just balanced with things further out.

The monster red-leaf "shrub" at the left looks smothering to me. I'd make into a tree as it looks like you're doing with the one at the right side of house. Extending a full bed of the light color hosta below it could be handsome. At the other side of the house with different light conditions, use a groundcover that will work there.

I think you need to place low shrubs in line with windows and then allot them much larger space so they can grow to a size that is coordinated with the house & window sets. Use some annuals to flank the entrance for color. Big color for that house and slab. BTW, the slab is a decent size. But the walk to it seems undersized. I'd add a strip of (probably contrasting) pavers along its edge in order to widen it without it seeming like an add-on.

I think a better place for the urns would be to the back, outside of the slab.

The red leaf "shrub" (at the far left of picture) I'm 100% sure is not a silver Maple. It may be a Japanese Maple.

If you want ongoing color it would most likely be an annual. If you settle for periodic color for a certain duration (like a month or two, depending) it could be a perennial. You'll need to take into what grows well in your area and what's available.

I disagree with Doug's proposal, but unfortunately can't compete with his graphic skill! I think the planting is too busy for your elegantly simple architecture. Your Japanese maple on the right is going to grow to fill in the space on that side. I would choose a low evergreen shrub like abelia or azalea and run them in a staggered double row across the front, with a unifying groundcover. No retaining wall. The slope adds interest. Now where I would consider adding "interest" is the expanse of wall above the garages (perfectly sited IMO, both for function and aesthetics). Depending on your zone, a vine or climbing rose?

The shrubs under the maple on the right (as you face the home) are dwarf Abelia.

I love Doug’s design, it really matches your lovely home and setting well (plus is nice usable space). Balances the house perfectly, and enhances the slope .... which, with no wall, makes it look like the yard is just sliding down to nowhere and focuses the eye on how much taller the garage side is.

I also like how Yardvaark trimmed the tree on the left, looks much better.