My vote would be plain subway. The range area is part of the general workspace. The design or motif might look a bit misplaced.

I think it's a very personal decision. If you've always wanted a design over your range, do one. If not don't.

Whatever you do, consider that you have a similar area, but shorter, over your sink. I think the treatments needs to stay in balance, and that may be hard to do with a frame. You may be able to do an inset, same color/different size or contrast, or a border if you want some textural variation or a touch of color. My subways have a border at about 2/3 height running all the way. I thought I might do something more at the cooktop, but just kept it straight, and I like it. We did go with a pot filler, so that adds another horizontal line in that area.

You have a nice finish and interest details with the valance and the glass front cabinet. Is there a reason you feel like you need more? If it is only because you think you are supposed to -- don't. If it ois something you really want, I'd say go with a light hand. Keep it simple.
Do you plan to put a plate/platter or something decorative on the ledge of the hood? If so, anything you did in the BS would have to compliment the items displayed there and in the glass cabinet without fighting or distracting.

Would it be possible to have some type of trim the same shape as the curve on your hood valance?

My kitchen's been functional for over a year, and still no BS, I haven't found what I want yet. (Cuz I really don't know what I want)

But, BS can make a huge statement. I think you need to decide what you want yours to say. Your 'bones' are simple and neutral, so the sky's the limit!

You have a very mono-chrome look going here. White subway would keep that going, but I think it might be a missed opportunity to personalize the space, and make it really shine.

For example, I think copper [plain or pressed] would be wonderful: true to the period you are going for, but a much less common than subway. Would bring some warmth and sparkle to the party.

I finally understood all of the posts about backsplashes when it came
time to choose mine too. Its really a big decision because it takes
the entire look of the kitchen one way or another. You either want to make
a statement with it or not. So I think thats your first decision, think about
your inspiration kitchens and if a focal point backsplash has always appealed
to you or not. Your kitchen is already beautiful, so just continue choosing
things you love! Good luck!
vic