The fact that there is a sequential chain of hashes makes it much simpler (although, non-trivial) to design the x11 ASIC. I have considered in the past a type of dynamic PoW algorithm that would change which hash function is used in a provably fair way. The problem with this is that now you have different types of attacks that can be used, withholding of blocks for instance to try to guarantee the next hash function to be used would be one that is already being calculated on an ASIC.

There might be a way to overcome this by having an independent difficulty for each hashing function, but if there exists enough incentive to withhold blocks by a percentage of miners in order to continue hashing using a more efficient PoW algorithm (in terms of power consumption), this type of scheme could fail.

A potential solution to this block withholding attack is to penalize this scenario, i.e. if the last 10 blocks were mined with sha256d, then the probability that sha256d would be used as the hash function for the next block would go down.