Makes perfect sense
We had a Legendary Pirate, then Legendary Marine, then Richest Man & now an Idol (Could even say she might be Most Famous Singer)
There is something interesting
Strong World revolved about Shiki who was gonna appear in Manga but Oda changed his mind & turned it into a Movie
So if we don't count Movie 10, then we have Z, Gold & now RED (Stampede is more of a Fanservice Movie, not Story Focused)
Zephyr wanted Dyna Stones (Weapon of Mass Destruction) to fulfill his Plan of Revenge (Power)
Tesoro became Richest Man to fulfill his Plan of Revenge (Wealth)
And now we have an Idol Villain
Kinda to how Roger was described in Chapter 1 "Fame, Wealth, Power"
So i believe Uta's Revenge Plan that makes her a Villain revolves around her Fame similar to how Tesoro's revolved around his Gold
In other words, the more Famous she becomes, the greater her chances of making her Villain Plan work
I'm guessing her Powers/DF & Plan require the most people possible to know her or love her or idolize her
Which makes perfect sense cuz Oda almost always gives Female Characters Hax DFs & not Close Combat-Based ones
Since her name means "Song" then i'm guessing that her DF Trigger when her opponent/People enjoys her Singing
(It's basically similar to Hancock's DF but instead of Sight/Affection, it's Hearing)
That's how her DF Powers trigger, but for what they do, i think since she doesn't look like Combat Ready & her DF is Hax, i believe she can make people defend her (Like how Fans defend their Idols). In other words, she can turn people against each other or make everyone who loves her singing protect her at all costs, this is why the more Famous she is, the bigger her Fandom, the greater her Army & the more she is protected & able to fulfill her Villain Plan whatever it is
As for her Backstory & relation to Shanks, the answer is also in this Fame
Zephyr was unable to protect his Crew & Family, while Tesoro was unable to buy his Lover's Freedom
So Lack of Power & Lack of Wealth is what made them become who they are
Meaning that for this Uta to have decided to become Most Famous Idol, she most probably suffered once due not being Famous enough
I'm guessing Shanks did something that caused her to lose something precious due to lack of Fame
Maybe she was supposed to sing one day to save someone (Maybe she wanted to reach some King or Royalty at some Kingdom) but Shanks had an incident there which caused her Show to get cancelled & missed her best opportunity, or maybe because of him she got mistaken for the cause of that incident & became a Wanted Criminal with a High Bounty which led to her having a very tough Life.
So she decided to fix this Bad Reputation & gain biggest Fandom ever as an Idol, so she can get her Revenge.
The Movie should end with a Performance in front of Royalty or very Powerful Kingdom, so SHs Opponents are gonna be the Strongest Royal Guards from those Kingdoms that Uta was able to control with her DF. While Luffy gonna fight her Personal Bodyguard + Her ofc