I dont know if this would be too complicated but what if when you purchase a potion like a legendary brew after that day, each daily potion stacks
So, for example
There are classes of legendary brews.. we will call them S,A,B,C,D
D being common items and older items -> while S is the newer items and more expensive items etc.
If you use a legendary brew on day 1 of your 7 day streak you are more likely to get a S potion on day 7
If you trow in a Legendary brew before day 7, you most likely get a D or C potion
Each daily potion stacks and makes the Brews more "Rare" to return better items
What does everyone think???