There has been proposal to change the PoW in case of potential 51% attacks from malicious miners during a fork. But such a change in PoW renders multi-billion-dollar of ASIC into worthless. which hurts economy so much and the average innocent mining users. I would propose, instead of PoW change, we could change the system to the same double sha256 PoW but mix it with PoS features. Such a PoW+PoS system has several advantages:

* It protects existing multi-billion dollar investments from innocent mining users,

* A malicious miner cannot launch attacks and rewrite the blockchain with 51% or even more hashrate,

* If we insert 4 PoS blocks between 2 PoW blocks, we'll have 2-minute block time span, that solves the long confirmation time problem,

* We'll suddenly have 5 times of block space, that solves the scaling problem,

* The PoS blocks only mine transaction fees, so the 21M cap remains,

* With careful design, the PoW+PoS transition _might_ be able to deploy with a soft fork.