First post on this thread. I came here after visiting the r/AshaDegree sub, which is, for the lack of a better expression, an absolute shitshow.
This is pure speculation, but given the evidence available, the planned abduction theory is the only one that makes any sense.
I refuse to speculate her parents or her brother were directly involved. ALL the evidence available suggests they weren't, and NO evidence in 20+ years has been found that vaguely links them to any sort of foul play. They would have to be criminal masterminds to conceal such a heinous crime AND to so firmly convince ALL investigators working on the case that they were always innocent, despite being the most obvious suspects.
I also couldn't ever believe that somehow Asha running away at night and Asha disappearing are not linked. What would be the odds of two similarly unlikely events happening the same night? I could imagine her running away and getting hit by a car, but you'd have evidence of a hit and run. And it still wouldn't explain why she'd have ran away in the first place. She could have ran away for no specific reason, which is already extremely unlikely in itself, but then for her to disappear with no trace? If she died of exposure or something, she'd have been found by now. Because Shelby is quite close by, therefore she'd have died in the woods in the area between Shelby and her house, and those woods have been extensively researched. In 20 years it's almost impossible that no one would have found her body even by accident. And what are the odds that she would run away inexplicably in the middle of the night AND run into a predator that was lurking about in the area by total chance? The Occam razor would just explode.
Finally, there's the book bag. We can assume it got tossed away from a moving car, but it was neatly wrapped into two plastic bags. Whoever threw it away had to know that the bag could incriminate them, and the safest way for them to dispose of the bag would be to just either keep it, or throw it into a river, or empty it into different trash cans, or set its content on fire, or whatever else that didn't involve the bag staying completely intact and untouched along a highway in the general area of Asha's house that was relatively likely to be searched by someone close to the family at some point.
Which leads to believe the person who put that bag in that place WANTED it to be intact.
And the most likely explanation for this IMHO is that this person wanted the bag to be some kind of...trophy. Which in turn is a very common behaviour among child predators.
So the only theory that makes sense in my head and combines all the (scarce) evidence that we have - at least the pieces that were made public - is that someone managed to approach Asha before her disappearance, lure her away from her house (maybe by giving her a false promise?), and then abducting her. If you think about it, if you were a child predator who has targeted a 9 year old girl who always stays with her parents, brother or someone else in the family, what would be the best way to get a hold of her? Probably abducting her at night, when she's alone, and no one can see you - and specifically abducting her at some location near her house, where she can get by herself but you don't run the risk of being heard or seen by someone who she knows, or who knows and can recognize you.