* Avatar: The Last Airbender is getting a live-action adaptation on Netflix: This is a major project, and could potentially lead to other spin-offs, including mini-series.
* The Legend of Korra, the sequel series, already exists: It's a full series, but could potentially be adapted into a mini-series format.
* There are ongoing Avatar comics and novels: These provide a lot of potential content for adaptation.
* The creator, Michael Dante DiMartino, is working on a new Avatar project with Netflix: While the details are still under wraps, this could involve a mini-series.
So, while there isn't a confirmed mini-series, the potential certainly exists! It's worth keeping an eye on future announcements from Netflix and the Avatar team.