Eye texture sampling in niagara, this is something that was really missing
Accessing all APIs impossible the way UE4 is organized (or else you got your own UHT solution as that Haxe thing seems ot have;p), but anything that is binded to Blueprint can be easily accessible in python or other scripting language (any language that object oriented programming and support classes). There was scripting plugin project but it kind of desolved in to void of forgotness because surprisingly there was not much interes in it.
And C++ is not that hard as people make it to, simply people now days are too used to higher languages but you only chain yourself locking there mind in to them and Unity users aspecially got there head locked on C#, you will always need C++ to do anything beyond what you can do in blueprint and UE4 in general because engine it self is programmed in C++