> a kind of sideways inheritance where you could override an ancestor in a package, but not have to override all the descendants to have them inherit the behaviour - really handy for third-party plugins

This sounds like the virtual classes that Tim were talking about ten years ago. (For the curious, the first programming language with virtual classes was BETA from Aarhus University.) They never made it into UnrealScript. His language research after that time was less incremental and sought to uproot almost everything about programming games. He moved away from object-oriented programming and more towards functional programming, specifically type theory. When I arrived in 2004, he was all about dependently typed languages, the big inspiration being David McAllester's Ontic. For a long time his plan was that his new language would be used to implement most if not at all of UE(n+1). Eventually that ambition had to be tempered by reality and thrown aside; I'm not sure if he's still working on programming language design.