Granted, it needs a good datastructure; I'm waiting for a friend to find time to discuss options there.
I'm firmly of the belief that beyond-wcoj is a question of implementation, not of theoretical suitability in practical scenarios.
So here's the problem. The core algorithmic step of "beyond wcojs" are "geometric resolutions". The core idea is to work with gaps in the space. So for example suppose we are joining two relations R(A), S(A) which is an intersection and suppose A is an integer domain. Suppose further than R's maximum A value is 100, and S's minimum value is 101. Then there is a gap of (100, \infty) in R's space and another graph (-\infty, 101) in S's space. If you "resolve/join" these gaps, you get a gap of (-\infty, \infty), which tells you in one operation that the join's output is empty.
On this simple query, this seems to work fine but if you have general relations (let alone non-integer data types) doing such geometric "resolutions" and finding efficient indices to index those "gaps" becomes quite challenging. But any good idea here will push the field!