You have a very good idea.
I am certain that once The Lion Guard gets to The Tree of Life there will be a new story-arch involving Rani. I just have no idea what Rani's role in that story-arch will be. I am also convinced that we will see The Lion Guard as Adults before the end of the series. How much of them as Adults we will see is beyond me (If it is more than one scene that would be awesome
). I really would like Kion and The Lion Guard To Return to The Pride Lands, but it all depends on how the series ends. Especially in rearguard to Rani's obligation to The Night Pride and The Tree of Life. She is described as being The Leader of The Night Pride, so if a romance happens between Kion and Rani I doubt she will just leave The Night Pride to go with Kion. Conversely I can't see Kion falling so hard for Rani that he disbands The Lion Guard and decides to never return home.
Assuming we get to actually see Kion and The Guard Return to The Pride Lands. I see it as being only a day or two after Kovu And Kiara's wedding. I just can't see Disney allowing Simba And Nala to become Grandparents. Especially not just as a footnote. Then there is the whole fact that The Lion Guard meeting the now Pride Lander/Outsiders is a story that I don't think should be glossed over (Which of course means that
if this subject is addressed it will indeed be glossed over).
In all honesty I would love even a gimps of anything Post Lion King 2.