One of the most interesting and promising layer 2s I have been researching is called StarkWare. They have their tech deployed in a working product on Ethereum already called dYdX exchange. Its really smooth, you should play around with it. I am not sure if this could be utilized on HIVE down the road for privacy...as you say I'm sure @smooth would know much more than me.

https://starkware.co/
https://twitter.com/StarkWareLtd
https://dydx.exchange/