A static list is trivial. It's just to loop through the list and rate each identifier with a probability score, and show them in order.
BUT when one wants a dynamic list, thigs get uglier. We already suffer from one sideeffect from the hazzle of determining wether a token is a function or not (namely the user-defined struct problem, where the arduino preprocessor won't generate function prototypes.), I do not think it should be thought of as a small and simple deal to implement a dropdown as seen in Visual Studio.
You need to parse all files, for all identifiers. And when this is done, you should remember what scope they're in, what arguments they take, if any. And the list continues on.
I'm not saying that this is impossible, but it's not easy either.
A dropdown for the core functions might be a compromise, but it will add another thing to the list of things to remember to modify between releases. Or, maybe we could use the identifers from the highlighting token list?