We are having an outdoor ceremony, and I believe a good friend of mine if playing the piano for us (he can play literally anything you put in front of him, he is insanely talented!) I of course would double check anything with him and see what is can play or is comfortable with.
Our ceremony will be outside on an island type of location at a pond that is then surrounded by woods on most sides, so a rustic style. And our backup is inside of a AC wood barn that is also a church.
Has anyone been to a rustic style wedding and had something besides the traditional wedding march?
Re: music for walking down the isle
https://www.theknot.com/content/ceremony-processional-selections
DH and the GMs walked down the aisle to "Till Kingdom Come" by Coldplay and I walked down the aisle to "Turning Page" by Sleeping At Last.
Check out the Piano Guys on youtube. The do a lot of covers of popular songs- maybe there is a way for you to get the music if you find something you like?
We didn't have a rustic theme per se, but our wedding was outdoors and our décor was very natural. I walked to this:
I just liked it because it was simple. I'm sure your friend could find a piano arrangement he's comfortable with.
Though apparently I'm in the minority.
ETA: for processional and recessional music, we'll do more contemporary stuff, like Here Comes the Sun and perhaps Can't Help Falling in Love.