I love all cosmetic type rewards in any content. That also goes for the leader board content, as long, as many mentioned, it doesn't create physical power gap between players.
Rewards could include, as already mentioned, cosmetic clothes, pets, but also other types of cosmetic rewards like housing furniture (trophies...), or stuff that can be added to guild towns.
Perhaps besides only skins, even new cool building types, as long as they again don't create power gaps of any kind. For example, gain a cool sauna building where your members can relax!
Guild store is cool idea as well. Perhaps it could add some "guild theme colored" spell particles pack. So members who contribute most for the guild, can get their spells changed to guild colors.
Perhaps that would only be available in guild shop if someone actually buys it first from the in game shop for the guild, so dev team has something out of it as well, so the game can get better supported down the line.