At the moment there's Quorum and Convention which encourage participation on meta and seem to be related (bronze / silver style.) However, there's no gold badge in this family - would this be a good idea to encourage more long term participation in meta?
I think encouraging more meta participation is a good thing. Adding a gold badge would complete the family of badges. Something like 100 posts with a score of 2 might fit it nicely. Not sure about a name.
However, there's no gold badge in this family - would this be a good idea to encourage more long term participation in meta?
In my opinion, no. I think a gold badge would cause badge hunters to swamp Meta with content that they wouldn't be contributing if there weren't a prize to collect.
The only long-term Meta activity that I find generally support-worthy for every user is voting. An additional Electorate-like badge for Meta voting I would welcome.
Quorum -> Convention -> Parliament
A gold badge would complete the set and prompt more people to pay attention to meta. But rather than just expanding the number of posts as suggested here, we should also raise the score bar. We don't want people to post a bunch of low-value stuff on meta just to hunt a badge, but we should recognize the people with steady, high-quality meta participation.
Convention is for 10 posts with score 2. For the to-be-named gold badge, I suggest something like 20 posts with score 10 or 50 posts with score 5 -- something that boosts the number and the score requirements. This is similar to the approach taken with tag badges.