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I think this would be a cool feature. The option to assign runes and masteries to a champion. So, if I selected Udyr, it'll automatically choose the jungle runes and masteries I assigned to him.
Likewise if I then changed my mind / the team wanted me to play adc instead, if I switched to Caitlyn, my runes and masteries would update accordingly.
Naturally I could still change my masteries and runes if I wanted, but I think being able to set default pages would be convenient and help avoid mistakes where people accidentally forget to change their runes etc.