Custom rarity distribution in draft

Being able to customize the number of different rarities in the packs of a draft tournament could allow hosting some spicy drafts, for example a draft with packs from random sets having 5 rares, 5 uncommons and 5 commons per pack.

UI-wise it could be enough to be able to specify the number of rares and uncommons in packs, and the rest of the packs would be filled with commons (plus the cards that are always included in some packs like snow lands).