PoW algo could be Hashcash, which has not been "exhausted"? Not sure what "usage varied" means? To further elaborate on the proposal, from the block start time to verification start time x the miner would run say Mersenne solver, then from x to block end the miner would run verification algo, i.e. Hashcash. The Mersenne solver would have to run with less than or equal to the resource usage of the PoW algo, and if unable to equal then the PoW could also for example run in background. The computation is encrypted and the network traffic is encrypted. So, no way to determine that the verification algo time allocation is being "varied". Gas could still be used to prioritize work, but the idea is that most of the time the PoW algo is not running, and some other more economic algo is running. The PoW is being used for Proof of Capacity, unlike the claimed Chia coin PoC that continuously uses the capacity in a PoW manner rather than proving it and then offering or allowing it to be used for some other purpose in an interim. If the issue is the desire for the economic algo to be able to run continuously, then the nodes could be split into 2 or more groups. For two groups, one would run from time 0 to x and the other from 0 to y with x < y with y including the block from the x group in the block y produces. The x group would continue from x+verify duration to x_2+verify duration and including the y block in the x_2 block output, and so forth.