What are some aggressive gambits that White could use out of the Nimzowitsch Defense? 1. e4 Nc6 d4 d5
1.e4 Nc6 2.d4 d5:
scores best for White according to my database and it is the move I play.
Your opponent can go into the French Defense with 3...e6, but this is not a good French with the N on c6 blocking the c-pawn.
4.d5 Ne5 is the most common.
Now White can consider Qxe4, h2-h4, f3... White has many aggressive options.
The safe 6.Qxe4 Nf6 7.Qa4+ Bd7 8.Qb3 continues to give Black problems. The pawn sac offer of f2-f3 seems to have clear compensation based on the development advantage it gives. But the problem with non-queen moves is that they allow Black to play e7-e5 which is close to equal.
a common line is 1.e4 Nc6 2.Nf3 offering Black to return to a standard opening with 2...e5. If 2...d5 you get an odd Scandinvian/Center-counter. Black usually plays 2...d6 and after 3.d4 Bg4 4.Bb5 a6 5.Bxc6 White scores better.