It depends how brief it would be. I don’t think there would be much to worry about Qui-gon being at the temple of the whills say in a flashback or something. It would broaden the Universe and make that association of the larger narrative.
The main point being where “Force Ghosts” originated from really. This strange mystical power that Qui-gon who was more or less operating as a bit of a maverick Jedi was perhaps one of the most progressive forward thinking Jedi.
The knock on effect really passes to Obi-wan and would give a tidbit of back story to Obi-wan’s actions when he fought Vader on the Death Star. It might ground Obi-wan’s actions much more but still serve as a surprise if you watched it for the first time after Rogue One.
I honestly think that is what they might be going for. Not trying to replace the main cast with these Cameos just bolster the narrative and grow the Universe.
When Qui gon said to Obi-wan “train the boy” which one was he talking about?
Neeson is also a very Solid actor, It would be yet another Good performer in the mix of an already solid cast.
That’s if the Rumor has any weight to it at all mind.
I am also starting to wonder how Chirrut goes blind or was he always blind?