This allows us to make any change at all in the Script language, like introducing new signature types like Schnorr signatures, which increase scalability by reducing the size of multisig transactions dramatically, or other proposals like merklized abstract syntax trees which is a research topic mostly. But there really are a lot of ideas for potential improvement to Script that we cannot do right now. This would enable it for free by just adding one more byte to all Script scripts.
@cypherdoc This development is a double-edged sword. It makes old nodes blind about what many transactions are doing, yet having this capability may be necessary for Bitcoin to compete long-term with alternatives like Etherium. I know that it would be nice if Bitcoin only ever needs to be just a peer-to-peer currency system, but IMHO this is not enough, and all the non-financial uses will become very important as the world economy moves to cryptocurrency.
So, to be honest in our perspectives we should ask "Would we support the scripting change if it was coming from a team headed by someone like Gavin who had a primary focus on main-chain scaling?"
If "yes" then we are not liking the message because of the motives of the originators of it, but also because it introduces a trust requirement on the majority client devs, and we have lost trust in them because they have allowed a full-blocks situation to occur and are partially captured by single company with its own motives and off-chain profit incentives.
Ideally then, a powerful scripting change like this should be preceded by an environment where no single implementation has a majority of mining and non-mining nodes.