what would be a good nut to replace hazelnuts?

Brazil nuts are much larger but the nut meat texture is similar, the taste is pretty mild. Macadamia nuts might be a good fit but be warned they are very rich and properly the nut with the highest fat.

I think walnuts are a good add to almonds. What else is in the recipe...that might help us help you

It might depend on what the recipe is. But I'd agree that almonds would probably be a safe choice. I'd add walnuts to that too. However, if you wanted to play with flavors and take a different direction, the world of nuts is open to you: peanuts, cashews, macadamia, etc.

Almonds would be my first choice.