Pantheon's established a trend. They can pull skin lines to fulfill niche and unique roles.

at first I thought they used Arclight Vayne and the Pirate skins because they're due for visual reworks and their normal look doesn't line up with their lore/origin, but TFT Pantheon's firmly from a skin line. He ain't a dragon in lore, but it allows him to fulfill an otherwise niche and unattainable role: Guardian and Dragon. This really opens the floodgates; in case of overlap (like Tryndamere and Olaf both being Glacial Blademasters most likely), they can pull from alternate universes. Lots of possibilities.