I most certainly support your suggestion to start up a Poniverse Newsletter. I'd love to read it. I voted that I would like to see it once a week, but that would only be if there was enough to write about to do it weekly. If not, I'd fine with it being every two weeks, or every month.
However, I do not feel that the newsletter should replace Canterlot Castle announcements. So, perhaps a portion of the newsletter could contain brief descriptions of new Poniverse events or features, and then link to a Canterlot Castle announcement for more information. This would be in addition to other portions, such as what you suggested - showcases of member created content. I also think it would be great if each newsletter also featured some kind of MLP related review, whether it be a review of an episode, a song from the show, or a fan work.
I also feel that there should be a new subsection of Poniverse Staff called Writers, or Newsletter Authors, or some other fitting title. I would recommend hiring four individuals straightaway to work on the newsletter. This way, at least two people could collaborate on the newsletter, and switch out with the other two for the second newsletter to avoid burnout and scheduling issues. The writing team would be lead by an administrator.
Lastly, I feel that the newsletter should be sent out via private message by Mailmare. This way, if a user is no longer active on this site they will not start receiving regular emails from us. Also, I feel that sending out the newsletter via email detaches it from the community, which could undermine its purpose of being not only a source of information, but another great way for the community to have fun together by publishing content to share with everyone else in the newsletter. Sending it out via private messages would make it more personal and closer to the community.