Quite simply, start sending threads in to admins/mods(selected specifically for this forum/role) and whatnot to be looked at before they make it onto the suggestions board. To help filter spam threads/threads of no redeeming quality(to make their jobs easier), a minimum letter count of 100 (quite lenient) must be met or otherwise itll be deleted prior to being sent in for review. Furthermore, you have to meet a minimum postcount of 10 and have been a forum member for atleast a week before suggestions can be made. This will help limit the amount of 1 post accounts making "we need more weapons" threads and said accounts from simply spamming in forum games for 10 posts and then posting it. I know the last one is harsh but I feel itll lead to a better suggestions forum in the long run, and they can still post, just not make suggestion threads.
Now, what will actually achieve threadhood? Easy, threads simply must be in depth and logically explain why it should be done as well as show that the op is capable of intelligently answering any questions concerning his topic if asked. If the thread is asking for something impossible, the group in charge of monitoring it could simply tell them that via pm. The DNSL shouldnot be strictly followed, as it stagnates the forum and forces well thought out ideas to be tagged onto the end of obscurity behind more often than naught horribly written ops with often different ideas altogether, where people are more likely to post about the OP than the idea that was used to bump it. While that sounds weird when im suggesting a less spammy forums, its only annoying and really spam when you get threads like this.
So TL:DR, make a couple people in charge of rooting out stupid threads.
Any questions? Comments? Concerns?