When you look at what's going on, Oda re-introducing Vivi now makes a ton of sense.
A: What was Vivi's arc about?
The idea of corrupt royalty being overthrown through revolution. In that case the royalty wasn't corrupt, but, Alabasta's revolution was a microcosm of what we're seeing now in the whole world; poor, starving people rebelling because they feel like they have no choice.
Vivi is the crew's Diplomat; she's a natural leader, who is bound to help the Revolutionary Army. It fits her character perfectly, and would show the Revolutionary forces that even a royal can abandon their crown and choose the people instead.
Imagine Vivi bloodied and broken, still fighting to protect innocent people, who were mocking her and attacking her for being a royal?
B: She's most likely wanted by the WG, and needs strong protection from them. I know Morgans took out a Cipher Pol agent, but, it was a low level guy, and Wapol isn't doing SHIT to stop Vivi from getting assassinated/kidnapped.
C: Oda is setting up the Straw Hats and world to have heavy connections back to the Void Century and the people alive during it (whether we like it or not). If Lily was an important character in the Void Century, and Vivi is Lily's parallel, we can expect her to similarly shake up the world.
WHEEEEEEEEEEEEEEEEEEEEEEEE IT'S HAPPENING!!!
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