You could probably post those in the
RuneScape General Discussion sub-forum, depending what the thread was about. It'd get some extra discussion going, supposing the initial topic was interesting enough.
I'd just try to not cross-post every single thread you find, though a broad post title of "Official RuneScape Forum - Game Suggestions" or something would warrant posting many different threads, ideally with some commentary and insight to why you'd like whatever it is to occur. Basically, as long as it didn't get too SPAMmy, it should be fine.