I don’‘t like “news” letters in general so don’'t expect too much of this post.
Critical bugs and fixes for them should be available for system admins but that’'s another issue and probably nothing for a news letter.
Six times a year should be fine, I assume that you try to have even less major Wildfire (and Spark) releases within a year.
“Summarizing the activity of the community for the past month” sounds nice … new plugins, new bugs, new feature requests, LG’‘s new points within the last two months, … a roadmap what we can expect to be in the next version, … new wiki articles or an official announcement that there’'s a wiki.
Probably also a short overview about the news within other Jivesoftware products like Clearspace.