The monero blockchain is 21GB as of 2017-09 on version .11
What ways are there to reduce this block size down the line? Wasn't RingCT suppose to make things smaller moving forward?
Is it possible to just take a hash of all the previous transactions and just start from that point in time of the blockchain in order to reduce the size?
You can't simply discard all of the older parts of the blockchain, because you need to be able to verify that every new transaction you encounter references valid outputs created in the past.
The vast majority of a Monero transaction is the ring signature and output range proofs. It may be possible to prune these from the blockchain after a certain period of time, which would dramatically reduce the size of the blockchain.
This is a matter of ongoing research at the Monero Research Lab, which has two full time PhDs working on it. They will report back in the coming months.
