Ah. "Code completion" has been often requested. But it's pretty difficult.
Difficult in which way?
Arduino have code parser, have a lexical analyzer, can find function prototypes and can open any source file? I believe nothing else is needed.
BTW, I have done all that in the past for custom designed CPU, now I would need only free time to study instruction set for AVRs and few MCU documentations in order to make my own compiler (at least for 328p and 2560 I'm currently play with)...
Making compiler I would rate pretty complex and difficult, not "Code completion" feature...
But it is fine with me if cannot be done.