I'd be content if they would simply show Ali once more. Whether or not she stays as a permanent resident of the Great Valley (which would change the series forever) isn't my main concern. I just hope they don't forget about her completely. I'm hoping the writers are waiting for an appropriate time to bring her back.
I do believe that, however, if Ali was to live in the Great Valley, it may or may not be taken well by the audience. After all, fans are used to the Gang as it is. Changing it up at this late stage to include a permanent 6th member, as loved as Ali is, I wonder if it would be "too much of a good thing."
Remember, "keep the audience wanting more." We want to see Ali again. Would this still be true had she been around all this time? Would she complicate the series? Would her permanent place in the series take away from the effect of the Gang as it is now?
Remember that the Gang of Five, has been together since the very first film. They found their way to the Great Valley together. I'm not saying that the journey they Gang had with Ali in LBT 4 wasn't as important. It certainly was an inportant journey, to find the Night Flower to save Littlefoot's Grandpa. I just wonder if changing the gang that started it all, would be for the best of the series.
I think I'd prefer to see Ali as an occasional guest character, maybe once or twice during the TV series, or perhaps in one more movie. Making Ali a permanent resident of the Great Valley may not be in everyone's best interest.