Circle or straight line appear to be the only RAW options.
Something that's not entirely clear to me is whether only the length / radius are adjustable, or whether the height and width are as well. That is, does the wall always have to be 20' high and 1' thick, or can it be shorter and thinner than that?
I've been playing it that you can only adjust the length / radius, but I've never been entirely sure if that's the right way to play it.
RAW says nothing about it being a straight line. Infer all you like, but Crawford seems to generally hold that RAW is confined to things that are stated explicitly. FWIW, Mearls says it can "zig zag".
https://www.sageadvice.eu/2016/09/10/on-grid-wall-of-fre-has-to-be-line-or-can-zigzag/
It's not clear to me if the "up to" only applies to the length / radius, or if it's meant to apply to the height and width as well. If the latter, it would've been more clear if they'd repeated the "up to". As written, it can be interpreted that the wall must be:
a) 0-60 feet long / 0-20 foot radius
b) 20 feet high
c) 1 foot thick
But I wonder if it should be:
a) 0-60 feet long / 0-20 foot radius
b) 0-20 feet high
c) 0-1 foot thick
Exactly. That's why I wouldn't allow wiggly walls. It's either a straight line or a circle.
Note that it's a 20 foot
diameter, not a 20 foot radius. (Which makes sense, since the circumference of a 20 foot diameter circle is 60 feet, to the extent that you are ok with the value of pi being 3. If you live in Indiana, this should be fine. *)
Here is why the wall has to be straight: S-bends could be used to catch a great horde of enemies in the AoE.
>v>v>
^v^v^
^>^>^
I'm having difficulty understanding the diagram, but I guess I understand the basic point. But how far apart are you envisioning the segments of the wall to be? In any case, assuming you are using a 5 ft. grid, I don't see how the "great horde" can be any bigger than 12 since the wall must pass through each creature's space, which has to use up 5 ft. of length. Also, they have to happen to be arranged in "firewall formation" (i.e., adjacent spaces). But maybe I'm missing something.
Anyway, it seems to me that there is a reasonable compromise between "straight line or circle only" and "anything goes". The restriction could be that the curvature of any bend cannot exceed the curvature of a 20 ft. diameter circle (that is, it cannot bend more sharply than that). If you are still offended by a wall squiggling back and forth, you could further assert that one side of the wall must be solely concave, and the other solely convex. Even taken together, these still allow partial circles, and even bending around the corner of a 10 ft. wide corridor, while excluding any sort of severe snakiness.
* Ok, ok, Hoosiers, it was 3.2, but whatever. Poetic license.