But then you’d want to match plurals: “there are… topics remaining” …I think?
Wait, no. Then we’d naturally stumble at “1 new topics” – even though “topics” means the set of unread and new.
Augh, after looking at this for a few minutes nothing feels quite correct. That sentence might be a booby trap. 
Again, these little hitches don’t bug me – but if I had to restructure it, it might look something like:
See [xx] unread topic(s), [xx] new topic(s), or browse other topics in…