This is truly a great idea and love it when I see casinos tryibg to get their users(customers) involved in their decision making process because to me it shows that the casino has some degree of respect for their users.
Bitcointalk members always loves bonuses just as any other human would love as well and I guess if you give the the option to suggestions, some wouldn't mind asking for a weekly as well as a monthly bonuse and to be frank, I would glad be in the matitude asking for both bonuses.
I really don't have any formula at head but I mosre of free spins and rack back will be great.
It's always a good idea to hear feedbacks from gamblers, this will help them to run a check on their performances with the gambling services they offer, this can be done occasionally as a routine program for the collection of various data and information for to do list/actions list, online social group and discussion platforms can be as well organized just as the one we had on the forum gambling discussion board.