I'm all for innovation and wild and crazy ideas, but sometimes it's okay to find a winning formula and stick with it, especially when that time is dinnertime. There's a reason beets and citrus are a staple of winter salads: They go insanely well together. But within those constraints, there's pretty much no end to the combinations of salads you can make.
With the citrus, I'm honestly not all that picky. I do like the bittersweet flavor of grapefruit, but oranges or pomelos or even bits of Meyer lemon would do just fine. The key is to cut them properly so as to minimize the amount of stringy pith that gets into the salad. That means either, or peeling and slicing it crosswise in order to keep those individual bits of pith short.
Pretty!