I've rewatched Journey Through the Mist last night, it's still to this day my favourite Land Before Time sequel.
I've always seen Ali as Littlefoot's romantic interest, even though we are talking about puppy love. For me there are a few reasons that explain this great Ali mythos that the fandom created throughout the years, the first was the promise of her return. We didn't have that with Chomper or any of the other guest characters. The other, is that besides the original movie, this was the only sequel to end with a flashback, but it wasn't about the gang's adventures, it was about Littlefoot and Ali, it was always about Littlefoot and Ali.
"Maybe we'll all live together someday..."
Every new sequel that came along, I always hoped it featured Ali. I was very excited when she returned in the TV series, I remember staying up all night to watch the episode the next morning after seeing the preview. I was disappointed by the way they portrayed her character, and because she had hardly any screen time. It sounded like they just brought her back to fulfill the narrator's promise at the end of LBT4.
I believe that Roy Allen Smith had a plan for the rest of TLBT sequels, and some that included Ali, unfortunately, we'll never see his vision of what TLBT could've been. I've always envisioned Ali as part of the gang, and as someone who would join them eventually.
I seriously doubt that she'll be in TLBT XIV, and we'll probably never see her again, even if they do make more sequels or even a new TV series. But if she does come back I hope it's in a way that honors her character in LBT4.
As for the whole cousins thing, Littlefoot's grandparents just use that word because they are fellow longnecks of the same species. They even say that they're a migrating herd and that they don't know who they are or where they come from. If they were in fact related they wouldn't have called the old one by that name, but by her real name, after all, there are no titles when it comes to family.
But even if they are cousins, that wouldn't be a problem in the animal kingdom, or even in animation. Remember that Simba and Nala were half siblings since they shared the same father

Do you guys think that we'll ever get to see Ali again? And if we do, we'll she be more like her character in the TV series or in LBT4?