By this I mean that only the last x blocks are saved in the blockchain (x sufficient high to be sure there are no forks).
Let me stop you right there! The only reason to store and keep the whole blockchain is to be absolutely sure it's the longest chain since the genesis block. Without the genesis block, which is hardcoded in any full client, you suddenly have to trust third parties again!
Think about this for a while, as what you're suggesting goes against Bitcoin's fundamentals.