Thats a very interesting point! Your right about the avatarverse being so great that you can make many stories and ideas from it. Although, Im pretty confident Paramount will make a Korra movie, unless Bryke sees an opportunity in this form of story telling that can expand their world in their own vision. We will have to see more confirmation that they will make one in the first place.
I wonder if Paramount is determined that the KORRA movie really be a KORRA movie, or if they'd be open to a different story set in the Avatar world. A 2 hour film could be the perfect medium to tell a smaller-scale story about a past Avatar (like an earthbender) or perhaps of a prominent non-bender. Though the audience wouldn't have established relationships with the characters, the Avatarverse is so rich that it seems a shame not to explore a small, dramatic portion of its history in this way.