I chewing on your suggestion about hybrid levels. That could work,except that they would appear on the Member Levels page, which would probably be confusing to members.
Also, as a thought about the subject; In my case I am building a membership site with membership levels for members, specialists, directors, etc. Members are likely to move between levels on a regular basis. For example a member may choose to be a member this month, a specialist author next, and a Super Duper, for the next, and then revert back to just a member. In other words the solution may be to make membership payments like a shopping cart with basic memberships, to which add-on subscriptions can be added without changes to the underlying membership.
So in the above scenario, the membership would be 'Member' with an option to add to cart a subscription (a membership level) for 'Specialist Author,' or 'Super Duper,' etc.
Hope that helps in some way.
Otherwise, great product.