Something like Primecoin who was using the hashing power to discover new prime numbers so the energy for mining was actually being used for something, but I don't know if finding new prime numbers is really that valuable.

The thing with PoW is that the calculations may not produce anything of monetary value. Proof-of-Work is essentially Proof-of-Sunk-Cost because the idea is that a block producers are kept honest by the fear of never getting their sunk cost back plus profit.

Is the idea of creating something of value then dead in the water in PoW? No, it isn't, but only if there is no market for what is being produced. As far as I know, there is no market for prime numbers, although there could be because there exist applications that need large prime numbers and their creators face the choice of generating them themselves or buying them.