Tataru's seiyuu sounded familiar to me so I tried to look up her name and the first page that popped was
this one.
Translation:
"Tataru < > Nanamo!? Theory that Tataru and Nanamo secretly switched"
-"Tataru's voice actress is the same as Nanamo's I think. I remember thinking that in the final movie their voices sounded the same."
-"It wasn't Nanamo who died but Tataru. Up until Costa it was the real Tataru, then after that they switched places. Your goodbyes with Tataru at Costa had hidden meaning. The voice actors are the same too."
I know some people have brought this up as a joke but I think it's really true. Let's look at the evidence.
First and foremost, it's physically simple for them to switch places. Their similar heights, builds and voices would allow them to switch quite easily with a bit of glamour magic.
Next would be Tataru's mini-mission of course. Why give her all of that if something wasn't going to happen to her? Why that "farewell, oops I mean see you later" scene with the long gaze? (Nanamo similarly gives you a long gaze right before she's poisoned). Tataru focuses on becoming strong and doing something she's always wanted to do in this mission. Perhaps because she knew it could be the last time she would have a chance to do it and she wanted some extra courage.
It is pointed out several times that Tataru has many random talents - maybe resistance to poison is one. But that's a stretch. What's more likely is that those pearls she scavenged at Costa are an effective antidote or deterrent to poison - the favorite way to commit regicide by sneaky assholes since time immemorial (they are supposed to "keep people safe" after all). As much as fans have been crying out for blood, even Scion blood, this could be one way to do it, assuming Tataru didn't survive. But I think with the pearls she could have.
According to the final cutscene, "Tataru" was in Ul'dah markets while shit went down in the palace - Nanamo would want to be close to watch what happens. Assuming Flhaminn and Urianger are in on this as well, they could have been the two in charge of escorting Nanamo to safety.
What the actual motive behind Tataru and Nanamo switching is is trickier. My theory is that Nanamo strongly suspected an attempt on her life and gave the Monetarists the perfect opportunity to do so by throwing the party and calling in a guest (a person who could be easily framed for the crime) to attend her privately. She arranged to switch places with Tataru sometime before she called your character to come see her. Nanamo also knew simple abdication would just put the city in the hands of the people who had been running it anyhow (the Monetarists) regardless of her attempts to put a better power in place - she has no real power at this point and she knows it. She has a plan to save Ul'dah, but it would be far too risky for her to take the attempt on her life herself so she needs a body double who is willing to die for her and let the Monetarists think they've won.
I imagine she will come back as a symbol of hope and a martyr to the people - especially Raubahn - and Ul'dah will be taken back by the two of them in a blaze of glory. And then they ride off into the sunset together. Or something.