It's not clear that any of the examples suggest a compelling need for a "complementary forum." Let's see how they might fit in to what we already have.
Announcement for releasing something (e.g., a new release of software x that comes up daily on GIS.SE).
In many cases this can be handled with a community promotion ad.
Brainstorming for approaches to implementing some new functionality (e.g., "any ideas of how can we leverage redis's PubSub in GIS?") and then building up on each other's answers.
This is the idea behind community wiki. Carefully presented, such questions are on topic here. CW allows people to provide partial answers that are gradually improved.
Organizing a local group meeting/conference ("who is attending [x]?").
See the community promotion ad link.
Getting people interested in working on a project together ("anybody want to solve [x] with me on GitHub?").
That sounds like a quick chat followed by a mass migration to another site to deal with that issue. It might qualify for a community promotion ad (q.v.). It can also be conducted, with some care, on SE itself. For a model, see an early effort on a sister site, the polystats project.
Asking for opinions about a particular GIS topic/event/technology.
Chat is a reasonable vehicle for this. No main SE site would host such a discussion: that's not the SE model or purpose. If, however, you're not just kicking back for a nice warm chat with buddies during your off hour and you are truly seeking information, then you probably want well supported, well crafted opinions. We're the site for that.
