I really like the idea of a tiered reward system, however I would make it by the day.
I would do a 1-10 tiered system, breaking down the current apy so that those who are staked for the shortest time get the lowest apy.
Tier 1 would be those that only stake for 24hr. (perhapse something small like a .5% 5day roi)
Tier 10 would be for those that stake for a minimum of 10 days. (this would be the maximum apy reward, 9% 5 day roi, or whatever the current 5 day roi is.)
This would incentivize people not only to stake but to remain staked for at least 10 days to get maximum reward.
If at anytime a person unstakes, the tier level would reset back to tier 1, for that person.
It would also be cool to use caterpillar from wonderland with his hookah for the image. using the hookah for the measurement of what tier you are currently on. Dividing the image up into 10 sections perhaps black and white or something, and colorizing it as you advance through each tier. This would add to the marketing of the whole wonderland idea.
Just some thoughts.