Alternate PoW's.

Would it be possible to have multiple PoW's, where the difficulty reset of each PoW algorithm was dependent upon how quickly they resolved the next block?

Each node has the logic to validate multiple PoW blocks.  At the difficulty reset, the timing for each PoW is calculated, and the difficulty for each PoW is reset based upon how quickly they found blocks for that PoW.  Then, you could introduce other PoW's into the system without negatively affecting (at least too much) existing miners of a specific PoW.  You could soft-fork in a new PoW (like keccak?) which already has miners using it.  I suppose you could also soft-fork specific miners out.  What are the chances of a single miner being proficient in all algorithms?  Not high I imagine.

This has to have been thought of before?