If the "shake up" is truly Jarvan III dying.
What I personally always wanted out of a new Demacia LU was a dive into the Lightshield lineage, specifically the four generations of Jarvan. There's not a lot known about them, nor their historical impact and they're all pretty plain without personality. We're told they are great people but have superficial evidence of so. Yet there is obviously a deep well of lore that could be drawn upon here because they would cover about 200 years of history and world building. Which would be phenomenal to knock out all in one lore event.
What we do know is the first Jarvan (supposedly the most legendary of them all) had a fool's war that likely crippled Demacia's small army and embarrassed them in front of their enemies. His heir Jarvan II is literally unknown, not even the old lore mentioned him, we could have stories of him building up the Jarvan's reputation within the kingdom but who knows. The third Jarvan heir is predominantly unknown too, where he does have some relevance to certain champions but most of that is stuck in the 'old' lore, so we dont know what kind of changes might occur. J3 is probably a borderline fascist or maybe a bigot at best, that's all I could assume about his character in the new lore setting.
Also, I know King Jarvan 1st has a prominent connection with Sion and would love to see that story around their conflict revised, and maybe a Sion bio update with that. But by no means am I holding my breath at that chance.
What I would do is kill off J3 with this new champion because he'd see the king as a symbol of the worst aspects Demacia has to offer, that would cause a pretty significant domino effect within the Demacia social structure that could be hooks for future stories. While releasing other stories featuring the history of the Jarvans to perhaps make J3, even J4 a little more sympathetic. Because without knowing much about J3, I can't imagine anyone would be sensitive over his death, In fact most might believe J3 kinda has it coming and sympathize with the terrorism of the new champion.
I definitely don't believe that any of my thoughts is what's upcoming from this particular LU event. I would predict that the remaining biography-less champions will have their's updated and their stories will be separate from the new champion's. IDk if there will be anything more to it than that. With Kayle + Morgana's rework following this new champion, we'll likely be spending quite a bit of time with Demacia so who knows what will be revealed in the coming months.