Why not have it set up where there is a fee scale based on how long you keep your coins on the chipmixer system? ranging from investment return for keeping your coins on for a long period, to paying higher fee when you remove your coins immediately. something like this.

+3 months= +1-2%
2 months= +.50-1%
1 month= +.10-.50%
10 days=  0%
5 days = -0.10- -0.50%
1 day= -0.50%- -1%
immediate withdrawal to 1 day = -1- -2%