*after 2.Nf3
What can Black do after 1.d4 Nf6 2.Nf3 ?

Those two moves by white are the most problematic I know of. I have thought about this for years. Black's difficulty if he doesn't know his opponent is that he doesn't know if he is a boring d4 player or will come out and fight with 2c4.
I'm reacquainting myself with the Grunfeld at the moment because I would rather 'risk' playing against an adventurous player than a London/Colle player with 1d4 Nf6 2Nf3 g6. I believe the fianchetto is good against unenterprising players. After 2Nf3 we don't know white's intention.

I would think you'd want to establish your position in the center, so either d5 or e6. Then either Nc6 or b6 next.

The problem with the Benoni, both old and new is that if white is determined, he can get his stodge on the board. Instead of pushing with d5 he can defend with Nf3 leading to a Symmetrical English or e3 / c3 leading something like the Symmetrical Tarrasch or Reverse Slav - not good, but not bad either, where a black player can struggle to win against a marginally weaker player than himself.
There are determined stodgy players who don't care whether they win , lose or draw as long as they can be boring. I'm sure their must be an underground book on this strategy circulating amongst players who can't get girlfriends and are looking for ways to upset normal players
Maybe consult:
Grandmaster Repertoire 11: Beating 1 d4 Sidelines by Boris Avrukh (2012)
https://web.archive.org/web/20140627001415/http://www.chesscafe.com/text/hansen164.pdf
I'm not putting a question mark after Nf3 cause I think it is a bad move, instead it is because it is a question. Anyway, I have been struggling against this move order, so please help. And whatever I do I will play 1. ... Nf6 so please tell me what should I play.