I guess that's always the problem with a competition. What do the judges want to see? What's going to make them take notice of my work? Should I do what I want or should I play to what I think others want to see?
I've been a little hesitant to comment on work for this competition as I really don't want to steer people in one direction or another so I tend to keep my crits short and more to an overall sense how I feel about it globally, but I will say what I'm looking for on all entries, and that's a keen attention to detail, good composition, a high degree of realism, capturing the essence of the objective of the effort, which is Iceland, telling a story with the image, and of course a good sense that the artist knows and understands how to use TG to do all this.
If you like your aspect, keep it. It's not like there isn't a huge body of existing work out there that uses a square aspect ratio. I'm more interested in how the elements in that aspect play against and balance each other. I'm more interested how the color hues all work together. I want to view an entry and be dazzled with the idea that I'm looking at something that I know is a render but wow, it really looks like a beautifully composed photograph, one that a landscape photographer spent time scouting out, carefully composed the shot, and then waited until the exact right time of day to fire off the shot. That's what gets me excited about this.
I will say on a detail level, and it seems that you already agree with it, that Dune's comment about the clipping snow highlights should be addressed. In general it's best if you can represent the total tonal range of a render without a lot of clipping as it tends to make something look like it was photographed with a cheap digital camera. I've spent lots of time in CG trying to avoid these occurrences and you'll likely have to address it in post to some degree. I see it in a lot of renders, including my own and it always bugs me.
Anyway, enough of my rant. Good work and keep going, and good luck.