Does anyone have unique ideas for a retaining wall?

https://www.backyardboss.net/retaining-wall-ideas/

Layered railroad ties fill with your favorite perennials and annual flowers

Stacked stone can be really nice. You want something structural as well as attractive depending on how much you are shoring up

Unique? You could dig a moat and use the wall as a circular dam. But a good wood fence made from railroad ties with mounds of rock and dirt on the down side is a strong wall. If you can dig out some of the hill you are trying to hold back, that will remove as much as a ton of weight from the wall for every none square yards, depending on the composition of the soil. Another unique idea? Stand a row of fancy cars headlights first in the dirt, like at the Cadillac Ranch in Texas. Nobody will have a retaining wall like yours. ☺️

Gabions - Clinker Blocks- Crazy Paving Stacked in a wall pattern - Rail Track Sleepers - Pantihose filled with soil and knotted to make bricks - Plastic Bottles filled with water on their sides.........

Not sure of the "look" you're going for, but I've used stacked field stone, just don't stack them straight up & down, you want them to tilt back a bit to prevent them from falling, & you might want to use a landscape fabric behind them to prevent weeds from sprouting between them. Myself, I didn't use the fabric & have had few problems with weeds, but if I do, I just yank them out.

Another thought, buy 80 pound bags of "Quickcrete" (spelling?) cement, either stack the bags in place, or lay them somewhere, hose them down a few times a day for a week or so, then rip the bags off, then put them in place. If you stack them in place still in the sacks, use a utility knife to remove the paper after a month or so. A concrete stain could be used on them to make them look more like rocks, or try growing moss on them using butter milk. Use some fresh green moss, the butter milk & an old kitchen blender, whip up a few batches & sloppily paint the mixture on the "rocks" & lightly mist to keep moist until the moss grows. The moss idea may not work too well if being used in an overly sunny spot. Good luck!

What about a retaining wall made with terracotta ag pipes ???

Whatever you decide, take your time and do it properly, otherwise your project will fail in a short time. Here is a link to build your wall stronger so it will last longer. https://www.familyhandyman.com/landscaping/retaining-wall/how-to-build-retaining-walls-stronger/view-all/

Also, I used weedblock against the wall to keep weeds from creeping thru. Good luck!