Sounds like a wonderful pond!
I know a lot of people have rocks around their pond edges, but I'm not a fan of that... doesn't look natural, and it's hard to access the pond to do maintenance or to just look at it. Flagstone would be a good choice, IMO.
If you could figure a way to get grass to grow to the edge, avoiding the problems proposed above, that would be a really natural look, IMO. Maybe you can put some kind of a concrete or concrete block ring right at the edge, and let the soil / grass come up to that ring. That would eliminate the wicking potential. You want your liner to extend as far as possible out from the pond, under the rocks / soil / grass / flagstone / whatever.