One Feed per forum for all postings. Results in jumping to the thread at the specified posting (not sure if forums supports this, but it is just one anchor to add, so I'd fix it).
One Feed per forum with *new* threads.
One Feed for *new* threads in *all* forums of a package instance, with entries named: [Forum]: Thread
How about a way to tie RSS into notifications? Instead of choosing Email notification, choose RSS. Then you have a single RSS feed comprising all of your notifications.