I don't think we're ever getting much from the Keyblade War but what chi is kind of doing and probably a cutscene in KH3 since we will most certainly get a quick rundown of the tale by somebody to the main cast since they really have no clue about it. We could easily get Xehanort backstory in KH3 as well. I don't think we necessarily need it besides maybe a visualization of Xehanort's Reports. A flashback of when he first arrived at LoD, his time with Eraqus, his disappointment in not being made Master, his journey in the Lanes Between without his armor, the quest for knowledge. This can all be done in a quick montage.
We're never going to see or hear much about the truth of the war. It is essentially a fairytale in the current time of the series. We're never going any farther back in the story than BBS.
A BBS movie, though? Too confusing do handle the three different perspectives and stories.