The DotNet Foundation maintain the System.Linq.Async NuGet package that is a set of interfaces to implemente async LINQ query provider.

MongoDB driver should implements these interfaces.

Proposal 1:

IMongoQueryable<T> should extends System.Linq.IAsyncQueryable<T>.

Proposal 2 (I think is better):

Add extension method to IMongoQueryable to get an IAsyncQueryable implementation. This approach allow to maintain the extension method and provider implementation in other project to avoid add a new dependency to driver project.