Your idea is a great one! In a perfect world.
As all the others have said, there are money costs and its THEIR story! You go ahead and try to contact Bryke and see if they can make a alternate ending for the shippers, but there's another thing that would make this impossible, and that is:
There are more than 2 ships!
Kataang and Zutara may be the most prominent as far as we can tell, but whose to say that IF they mad an alternate ending that there won't be other ships that want one too? Maiko may want an ending (if it isn't shown in the original), Sukka or Tokka may need one too if it isn't cleared up in the show's ending.
Bottom Line: You Can't Please Everyone. That's What Fanfiction is For!