When I looked through the RCode list I found four codes already assigned to minor disticts in Chiang Mai which were not yet created. At first I thought these are district once planned but than scrapped, but just to confirm I picked one of the names to Google to verify - and did the lucky pick because that one seems to be one which is about to created in the near future - though I haven't yet found any fixed date for the official creation.
The code 5026 is listed to belong to กิ่งอำเภอวัดจันทร์ (minor district Wat Chan), and is supposed to cover the subdistrict Ban Chan (ตำบลบ้านจันทร์), Mae Daet (ตำบลแม่แดด) and Chaem Luang (ตำบลแจ่มหลวง) of Mae Chaem district. The name seems to be not yet finalized, as I also find it as อำเภอวัดจันทร์เฉลิมพระเกียรติ (Wat Chan Chaloem Phra Kiat) or just อำเภอเฉลิมพระเกียรติ (Chaloem Phra Kiat). The latest of the news reports I found are from January this year, however the whole project has started quite long ago - Chiang Mai News had a report which lists the district history back till 1993.
Another news clipping suggests even more names - อำเภอกัลยาณิวัฒนา (Kanyani Watthana), อำเภอราชนครินทร์ (Rat Nokrin) or อำเภอกัลยาณมิตราจารย์ (Kanyani Mitrachan), all in honor of the Galyani Vadhana, the late sister of HM the King.
I am also not sure whether this new district will be created as a minor district (King Amphoe, กิ่งอำเภอ) like it is listed in the RCode list, or directly as a full district in line with the 2007 upgrade of all minor districts. Still quite a lot of questions and probably still a bit time to research before the creation get imminent, but now I am forewarned I doubt it will be same as for the last new district of Thailand which I only heard from about a year after it was created.
For a more entertaining link, On the road Wat Chan - Kun Chang Kian is a great photo album of views on the area which will make up the new district. And to give due credit, the map above is based on the new vector maps hdamm is creating for Wikipedia, replacing the more plain maps I created years ago.