Support for tags
It's very common that a Thread would fit into several Forums or at least is of high interest to several different teams.
It'd be ideal to be able to use tags to tag threads and that user could subscribe to tags no matter in which forum they are in.
In product and engineering teams this would be very convenient for cross dependencies.
It'd be incredibly useful for example when Account Management works side by side with product around a certain account (which would be the tag).