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](https://gis.meta.stackexchange.com/questions/540/community-promotion-ads-2012).

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> 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.

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> Organizing a local group meeting/conference ("who is attending [x]?").

See the community promotion ad link.

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> 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](https://stats.meta.stackexchange.com/search?q=polystats).

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> 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.