Proposal: Execution Order "Layers"

Imagine a scenario where you want to create an unspecified number of generic systems at run time/start up, that need to run sequentially in a deterministic way.
You can't use the UpdateBefore/After/InGroup attributes because all the systems are of the same type with an unknown generic parameter.

In this situation, it would be really helpful to have an integer based execution ordering system so that you can specify at run time which instances should get updated when.
So all instances in layer 1 would get updated before any instances in layer 2 and so on.
This could be a valuable extension to the system already in place.

But maybe I just haven't tested out the system execution order features enough and there already exists a solution to this problem. If so, it would be great to hear it.