I like it but the problem in what you suggest is that there is no way to prioritize certain roles.
Don't you think this would be a better idea?
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Instead of having to choose only 2 roles, we should have the possibility to choose up to 4 roles.
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Every players must select at least 2 roles out of 4 in order to enter the queue. (It means there would be 2 optional roles)
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The roles must be entered by order of preference from left to right.
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If a player select "Fill" on a certain role, he would not need to select any other role.
Examples:
ADC, MID, Fill, Empty
MID, Supp, Jungle, TOP
TOP, Jungle, Empty, Empty
This system would cover efficiently any possible combination of roles a player want to play and would also greatly reduce queue time.