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.
Aside from data gathering from users, they are doing this too as a promotion for their casino aside from their ANN thread. Engaging on user here might convert to potential customer in the future thats many casino always do a survey, review and promotion that benefits both user and casino.
Free spins and rake back are the most common bonuses given by the casino. They only varies on the amount of spin and percentage of rakeback. I think having a free bet on any games without any wagering requirements is the best promotion in exchange for a certain activity done in the casino.