These are all good ideas, but way too much work to implement. Mission rankings are a lot more work than they look, since I'd have to implement a timer then personally determine every single rank threshold for every single mission.
I've purposely avoided achievements because they're also a lot of work for not a lot of returns: for a speedrun achievement, I'd have to implement a timer, for X sprayables/X grinds in X way I'd have to implement entire new scripts, and so on. Then I'd need to implement an entire achievement interface to handle them. It's a story for another time, but I also never really liked the "culture" of achievements, so to speak. It might happen, but don't bet on it. I'd rather work on bugfixes and E2.