What should be my next move as black?

So I was training with an ELO 2100 but I'm stuck. 

What would be the next best move? 

If White isn't doing anything, I would also do nothing.  A draw would be a good result for Black.  This was not a good opening for you and your position is awkward.  The only active plan would consist of ..f6 and play on the halfopen f file.  But this has the drawback of giving Black a backward e pawn, which could be a significant weakness long-term if Black is unable to get decent play.  Not to mention you've played ..h6 so g6 would be a potential outpost in the vicinity of your K.  Typical French middlegame stuff, I'm sure you've heard this before.

Rook sac!!1!1

(Nf5 might be interesting)

Yes there are lines in the Advanced main lines where Black can play ..f6 with the idea of opening up the f file and exchange sacrifice on f3 to take apart White's center, but my instinct is his position is not good enough to support this.

As I look at this more I realize that 20...f6 (and, because of en passant, 20...f5) would just be flat out bad for Black due to an immediate Ne5:

21.exf6 Rxf6 (else h6 drops)

22.Ne5! Nxe5 (else f2-f4 next)

23.dxe5 Rf8 24.Ne2 and this N will become a giant on d4.  If 24...d4?! (desperate attempt to open up the d5 square) 25.Nxd4 Nd5 26.Be4 threatens Rxc8 and also f4-f5 which crushes you.  Otherwise just Ne2-d4 and White expands his 4-3 Kingside advantage while keeping the blockade in the center.

In short, your position after 20 moves is not good.  You need to look at what happened in your opening so you can do better.  I can tell you your 5th move is an inaccuracy and your 8th move is illogical - you let your opponent get in a3 & b4 for free!