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As I've said before use weekly queues. Depending upon how many champions you want people to have you do extended rotations, so a weekly rotation is up for 2-3 weeks or however many heroes you want. Each week the oldest week gets moved out of the rotation and heroes from the new week are added.
This gives people time to get accustom to heroes they've never played, allows them to play with a selection they know, still adds value to riot selling heroes (which is lost if everyone gets all the heroes), still allows riot to showcase selections, and allows for a semi-balanced pick as per weekly pick.
The all heroes selection idea is stupid for a lot of the aforementioned reasons (like people having absolutely no experience with all the heroes and never getting them again).