I suggest the above because no one really wants to check 1000s of threads to see if their suggestion has been suggested before. If lots of the key suggestions which are overused (or just ones that have been suggested before) are put into one long thread then you could simply search for your suggestion in that thread (using Ctrl + F) to see if it has been suggested before and spam is prevented. All it would require is that an admin maintains the thread (to justify what should be included in it) and all he/she would need to do is keep an eye on the suggestions forum.