Well, two main things are the custom storage layouts and a delegatecall mechanism.
Layouts (beyond the rudimentary support we already shipped) are work in progress. The implementation part is easy, the main blocker is still the time needed to properly design it to address all use cases. Depending on the demand, that feature may still ship in Classic Solidity (more details will come with the roadmap wishlist).
As for delegatecall - that’s a tough one. We’ve long been dissatisfied with libraries and planning to drop them in the transition. The problem is that a replacement mechanism is still an open question that we’ll have to figure out.