I was thinking about the issue we always have with looking at a given business situation and determining whether Spaces or Groups work best to implement a solution. Groups are often a better fit - more flexibility and granularity in setting security, more ability for the end users to manage the environment themselves. On the other hand, Spaces seems to have more tools for consolidating information from subspaces.
Would it make sense to be able to define trust relationships between Groups? The idea would be that if two groups trust each other, then they could share information, have widgets that display information from trusted partner groups, etc. This would let us create hierarchies (A trusts B, which trusts C1 and C2, etc.) or many to many relationships (A trusts B, C, and D; B trusts A and D; D trusts A). Yes, you might need to be careful not to create loops (A trusts B trusts C trusts A), but there are algorithms for that. (And there might not be any issues with such a loop, anyway.)