I don't understand why anyone can't modify the data structure you're calling the "next block" to change the transactions to arbitrary ones he chooses. There is no "difficulty" to your process of block creation. One could go back an arbitrary number of blocks, modify a transaction, then recalculate the chained blocks going forward to the present and pass it around as the real block chain.
Any new idea must make it "difficult" to create a block.
Sorry. I thought the difficulty currently is the sheer amount of hashing required to find hashes that are less than some value. What other difficulty are you talking about?