Your first point (Avatar being cut short): It seems your trying to convince us that avatar was finished too quickly, when in reality we've known since the beginning of book 3 that it was going to be the last one.
Actually we've received hints of it being the last one since book 2, so i don't know why you haven't figured out by now that Avatar was not cut short.
Your second point (Avatar ratings being the reason for it being cut short):
As for the ratings of avatar, it's Nickalodeans most successful show to date, so you can't say:
"ALTA was originally meant to have four seasons, but was cut short by the Nick execs who didn't think the ratings were high enough."
People have said for a long time, it's Nick's greatest show ever, or definitely in the top 3.
So the cut short for ratings premise you have... doesn't hold up.
Go look at the ratings section on wikipedia:
http://en.wikipedia.org/wiki/Avatar:_The_Last_Airbender
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Your third point (Filling gaps in story) : I've read a lot of stories and been through countless adventures. My favorite types of stories to read are series because i get to enjoy the world longer.
If you've read a lot of good series that ended, you'd know, in every single one, there were loose ends. It's nearly impossible to finish off a series by tying up EVERY loose end and continuing the plot the way you want to. This is usually the case because authors don't have supernatural memory powers and it's hard for them to remember every loose end.
That's why Jk Rowling is answering loose ends, instead of writing more books, so she goes around listening for questions and then she answers them and we all get to tie up loose ends. Sometimes new things, that no one could think of, that only the author knew, come up after the series, like about Dumbledore and the wizard he grew up with and their relationship.The answer to that question came AFTER the series ended.
With avatar, even if we were given a new series, we'd have to be given a NEW plot, and with each new plot, comes NEW loose ends, so we'd need atleast 2 or 3 new series just to tie everything up and all that with a NEW plot.... that's very unreasonable.
If you want the loose ends tied up, i'd just ask the creators for the answers, or we might see the answers in the new movie.
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Sum it all up:
Avatar was not cut short, it did not lack in ratings, and the loose ends can't be tied up with a new season.
Also a new season, if done improperly, and at this stage of the show, might destroy the credibility and amazement of the first 3 seasons of Avatar.
I'm not ready to risk that, and for a show you love, you shouldn't be ready to risk that either.
Lets not be greedy, if you want solace or comfort, your only option (as an adventure seeker) is to just go looking for a new adventure.(A new book or tv series)
That is the cure to the end of avatar, not destroying the series with a horrible 4th season made on the fly.