Currently, target is stored in each block. Why is that? Why it is not simply computed by each client based on the previous timestamps from previous blocks? Why it has to be stored within the block?
In other words, doesn't allowing miner to choose the target remove that it is exponentially harder to create a new block? Other miners might have to mine 256 consequent blocks, but I have only to be lucky once and get one block with 8 more zeroes to replace those 256 blocks. While it is less probable, it grows linearly (256x less probable) and not exponentially.