I usually use almonds and canellini beans (I've found the wetter beans means it binds a bit better. Tried chickpeas a few times but it has ended up a bit crumbly). Like Ed said, you don't really taste them, they are just for the texture etc. You could maybe try oats? That's what I use to bulk out bean burgers (blitz them a bit in the food processor first), and in theory it would well work as a filling. I usually add a few tablespoons of ground flax to the mix too, which will help the binding as well.
I usually use almonds and canellini beans (I've found the wetter beans means it binds a bit better. Tried chickpeas a few times but it has ended up a bit crumbly). Like Ed said, you don't really taste them, they are just for the texture etc. You could maybe try oats? That's what I use to bulk out bean burgers (blitz them a bit in the food processor first), and in theory it would well work as a filling. I usually add a few tablespoons of ground flax to the mix too, which will help the binding as well.