Arrays. We need arrays. Not lists, but arrays. I'd like to be able to access a single element of an arrary properly instead of having to call the llList2List() function. This includes 2, 3, and higer dimmentional arrays. Although I don't mind if it doesnt' go higher than 2.
Class or struct data type itself is needed, makes for a lot of custom implemented data forms. The computer science people out there know what I'm talking about.
Pointers would be nice to have. combined with the class or struct datatype would allow people to create dynamically linked lists. If this causes too much ability for the user to cause havoc, try and find a way to limit the memory available to each object/script/owner to eliminate sever problems.