It’s probably a straight line (or a ring).
There are many wall of- spells in D&D 5e. Regarding shape of the wall, many use this formula:
You can make the wall up to X feet long, Y feet high, and Z feet thick.
Then, some of them go on to define alternative shapes. The following spells follow this formula (those defining an alternate shape will be denoted with a †):
- prismatic wall†
- wall of water†
- wall of fire†
- wall of light
- wall of sand
Each of these spells let’s you make a wall, and three of them define an alternate shape.
There is another wall spell, wind wall, which has a very important feature that is notably absent from these other wall spells:
You can shape the wall in any way you choose so long as it makes one continuous path along the ground
This phrase is entirely unique to wind wall. Wind wall uses the typical formula these other spells use, then it is uniquely defined as being able to make a wall of any shape.
If the intent of the statement:
You can make the wall up to X feet long, Y feet high, and Z feet thick
is to allow you to make a “wall” having any shape, then this unique feature of wind wall is entirely redundant.
There is room for an alternative ruling.
Obviously there is some ambiguity. My argument above requires understanding and comparing wall of fire to five other spells and saying “one of these is not like the others”. The trouble with ruling that wall of fire can be any shape is that this ruling says “even though one of these is not like the others, I’m going to rule they’re all the same.” There’s room for this given the ambiguity of what a wall is in this context, but I don’t think it is the right ruling given the uniqueness of the description of wind wall.