Like notification feed haves categories, hubs can have categories and be inside other hubs in collapsible list
One merged site can belongs to multiple merger sites and a site can be merger site for multiple type of merged sites, so would add lots of unnecessary complication
Unless you rethink well and change how hubs works.
A hub belonging to more than one site is the unnecessary complication
a hub belonging to more than one site is the only reason you can have multiple frontends for something like zerome