Add a whole immutable library of structures. * immutable List (tuple) * immutable Stack * immutable Queue * immutable Tree * immutable Dictionary

anything else??

There is already the ImmutableStack, and a list (Sequence).

For the immutable Dictionary I recommend using the ToLookup() Linq function, which creates a unmodifiable Dictionary-like structure.

For the Queue, and Tree we could again look to Eric Lippert's blog series on this topic, where he presents pretty sophisticated implementations.