We did this in the dystopia forums and it worked well.
Basically you could vote +2, +1 or -1 to a suggestion.
If a suggestion got enough votes, the devs could either confirm it as in the works, confirm that it was under consideration, or flat out vito the idea, usually due to engine limitations or the idea being outside the scope of the original game design.
If it didn't get any votes, or was voted down to a certain point, it would be locked and added to the "do not suggest" list.
Not saying all of this is explicitly necessary, just would be nice to have a clear scale for judging how popular, or unpopular a suggestion is.
ps. I +2 this. ;)