When you browse the actives discussions in "My Topics" page, the topics are sorted by forum name.
Why not using the Category SortOrder and the ForumSortOrder ? Forums are often sorted by relevance, so their last post could be sorted this way.
To apply this change you should just change the topic_active stored procedure from :
order by
d.Name asc,
Priority desc,
LastPosted desc
to
order by
e.SortOrder asc,
d.SortOrder asc,
d.Name asc,
Priority desc,
LastPosted desc