Given that there's no ultra wide in your current kit -- read this if you decide to get one. The 15mm limited is only barely into ultra wide territory, so it's more forgiving, but still worth a read. I think it's a great choice... if you're using it as it's intended to be used.
How to Use Ultra-Wide Lenses
Most of the time, Ken Rockwell is ... maybe not the best place to get advice... but his article on using ultra wide lenses is pretty spot on. The bit about using polarizers... eh. Try them with, try them without. If the sky is clear you can fix any banding in it in post. I think it makes up for itself with glare reduction on the ground and improved foliage and so on.
And of course, check out the 15mm and sigma 10-20 lens clubs for inspiration. Might even be some grand canyon and so on shots in there if you search.