The question of incentive becomes more of an issue should the best minter not mint and then no-one else wants to (although I think in reality this wouldn't occur unless the platform had been basically abandoned).
Perhaps you are thinking this approach has a Sybil problem but as account creation is strictly controlled (by a much harder proof of work that has to repeatedly occur at a regular interval) then it becomes increasingly more difficult to create Sybils (so POW is being used to prevent Sybils but accounts are being used so that intensive POW is only needed infrequently).
I imagined something similar sounding recently:
1. 'Identities' were generated by miners in a similar way to how blocks are mined in bitcoin, with increasing difficulty etc
2. These identities can then be used to mint blocks
3. The influence of a particular identity would be proportional to the 'difficulty' when the identity was created originally
https://bitcointalk.org/index.php?topic=1295981.msg13300342#msg13300342I concluded that this system has a similar security model to plain PoS, since identities never get 'consumed' like they would in a PoW chain. I'm not sure if this is similar to your design?