Its commented befoe (forgot who, but someone official) that A.net dev have people looking at forum for feed backs, and sometime suggestions too. Also in the Behind-the-Scene DVD, they do talk for 5 min of how A.net listen to the community, and cases of forumer becoming a Dev... (or something like that)
There are few ideas that is translated (thought most often not exactly) into the game that has its orgin from the fourm. And that list is long too.
But keep in mind the Dev would have other issue to concern about in regarding to a idea, so what is on the paper might not always be perfect in pratice or when coded, which might result in its change or not be use at all.
And I don't think A.net can really credit a fourm idea at all, thus hard to say what is the orgin, be it from such fourm or in-house.
But as a fourm suggestioneer myself, the question become what is the purpose of your suggestion? As this is not a A.net fourm, I would say a direct suggestion to A.net would not be the best attempt. Take it more as a discussion fourm, where you bring out a idea, and let other talk about it. If the idea is popular and good, and got many intersted and /sign, would surely bring it to Dev's attention, making it possible to be seen in the game. More importantly, take it as an entertainment, a place to share with other of your creativity.
However, few of the most popular ideas does strangely never got any response...*cough hairdresser cough* (and the coughing can go on)