Hello,
General idea is show all topics. I can add a param mids
Something like forum/rssalltopics.xml?fids=1,2,3&mids=1,2,3. Show topics from forums with id 1,2,3 and created by users with id 1,2,3
or add new settings on ACP
Inbuilt rss show the topics sorted by reply. Example https://invisionpower.com/forums/forum/431-ips-marketplace-topics.xml/
My plugin show the topics from selected forums sorted by start date