in my previous work, which you can find at https://github.com/p9c/pod I created a proof of work system with one algorithm but multiple targets to have multiple parallel block intervals operating at once. It occurs to me that higher proof miners could be submitting lower target shorter intervals - to change the checking - thing is though the check is expensive so idk. Anyway, point being that you could have several degrees of difficulty for different purposes like you suggest. Ah yes, and my algorithm uses mainly very large integer long division, which cannot be done faster than on a 64 bit CPU (intel, amd). So it also can function on lower spec devices with 32 bit dividers like ARM processors and GPUs.