Quintessence/WMG

Huey is the Thief is Ian
Does this even count as a WMG? Just about everybody has known or suspected this ever since his Heel Face Turn.

The big change Dasha Made through her time travel? Serai's existence
Serai sure seems to be nowhere in anybody's mind during the prologue. In fact, from the time you boot up the game, there is not a single scrap of evidence pointing towards Serai's existence until after Dasha pushes Reivier through the time portal thing. She's not in the flashbacks, she's not referred to once in anyone's dialogue, and she's not in the opening montage of scenes throughout the game. What if Serai's existence was the big change Dasha made? Reivier finds out Lunair's secret by being observant and suspicious after the event at the start of the story (only real difference is that it requires him to be even more observant). Without any real reason not to, and not entirely trusting her, Reives follows Lunair to Aeria to rescue whoever she was imitating instead of Serai. Everything else probably happened more or less as we already saw, though the love pill may've required a bit of a contrivance since he wouldn't be as devastated. The idea behind Dasha giving him Serai was that he'd have extra incentive to stay safe and alive (helps that they're so deeply in love/such great friends/whatever degree you feel like reading into their relationship). I guess this theory only really makes any sense if we assume that Dasha wasn't planning on Lunair replacing Serai but expected her to impersonate someone else instead, since his wife being kidnapped gives Reivier even more incentive to go.

Without Serai, there's nothing to stop Reivier from just getting himself more and more deeply involved until he eventually winds up with what we saw in the prologue. With her to hold him back, even with Dasha's massive miscalculation that caused him to still accompany Lunair to Aeria and still take the pill, he still might not dive the rest of the way in and find himself back in that chamber stabbing Lunair before Agatha can get her. At least, not in the same circumstances. Maybe they'll have a certain fourth person in the party when the grand finale rolls around again, and something she does will make a difference. And it's still not too late for Lunair to come clean and give him the antidote this time, seeing how he's being torn between her and his wife.

As a support, Reivier's lines from the last scene of the prologue: "I don't want to be a part of this, Dasha. I NEVER WANTED TO BE A PART OF THIS!!!" . . . "I. . . I don't want to ever be involved. I want to live my life. . . oblivious to what was happening around me. I want to believe. . . Believe every lie that I would tell myself. And I. . . I don't want to ever have met her." Dasha did her best, she just overlooked one tiny detail which turned out to be a very huge detail, that detail being the fact that Lunair had one more option when deciding who to impersonate in Korbin Village.
 * Except that if Serai didn't exist in the previous world, Reivier wouldn't have held back on loving Lunair. He still seemed to be treating her the same.
 * The love pill can be explained assuming that Lunair used this to make Reivier marry her while she was imitating a woman from Prion. It was also the reason why Reivier followed her to the temple instead of staying home as he was surely told to.

Dasha's big change was making Vikon search for the cure in Reivier's place
No, this theory makes absolutely no sense since that would put Reives as far away from Lunair as physically possible, but come on, if that wasn't heavily implied in the scene in which Vikon steals Reivier's call to adventure before he can jump at it I don't know what implication is anymore.

Lunair is being prepared to be a sacrifice in the same way Shanti was
Theory's too long for me to want to repeat it, see here.

The eyes in the intro are that underwater snake woman thing, and she will attack when the finale comes around again.
The former's pretty out there and mostly amounts to "She's the only 'monster' we've got so far". The latter seems almost guaranteed.

Lunair will give Reivier the antidote, but by the time she does it won't be effective anymore
Lunair's mother seemed pretty upset at the prospect that Lunair might've been given the pill, and was adamant that she drink that antidote right away. The pill she was given was an experimental suped-up version of the normal pill that they tried to give her because they "might as well" experiment on her since they were going to kill her before long anyways. And it's been months since she slipped that thing to Reivier. In chapter X she's still tearing herself up over whether or not to give him the pill, but it won't even make a difference--the window of opportunity for undoing the love potion is long, long gone.

The Quintessence's 'Venom' contains Small Bombs.
That's why the test results turn into a puddle of blood - THEY EXPLODE FROM INSIDE THEIR STOMACHS.

THE QUINTESSENCE DOES NOT EXIST!
Everything can be explained with Human Tricks!!!

Serai's a villain.
If the Dasha created Serai's existence theory is wrong, this could explain why Serai wasn't in the prologue even though apparently Serai volunteered to go to the pyramids with Reivier. She did a Face Heel Turn and reappeared as some random boss monster, laughing.

The Serai that came back from Aeria as a wolf is not Serai!
Serai is already dead. Kan Gao is trolling us by showing the Serai escape scene. Someone killed her, took her place, and used that chance to stalk Reivier for unknown reasons!

Serai is Dasha.
Go figure.

The mermaid, monster, sea thingy is not actually a monster, just a harmless sea creature.
After all, did it actually DO anything to Lunair underwater, except for swimming towards her? While I hardly blame Lunair for freaking out and attacking her (it?), it could have just been curious about her, then decided to follow her up to the surface, kinda like a persistent dog. Or something.
 * Well it did steal her purse, I think. Maybe it's an aquatic pickpocket! And at the end of the chapter all of its fishy friends got together and shamed it into trying to make amends... or maybe it decided Lunair had some really nice stuff in that bag and wanted to risk stealing it again. Maybe it's got a taste for giant mushrooms.
 * No, it was one of those squid things that stole the purse. Not the mer-man-woman-thing.
 * I thought it shapeshifted into the snake woman.
 * How does that work? The squid/mer-man-woman-thing has the quintessence?

Serai was Dead
In the prologue reality, Serai had been killed somehow, leaving only Lunair and Reivier to try and find the cure. The change that Dasha had made was that she no longer dies.

Lunair is a troll.
Yes. I suspect Reives' been trolling us!

Lunair IS 'Serai'. She created this character out of nowhere to spy on Korbin and wound up getting married to Reivier, because he fell for her shallow, kind and sweet facade. Lunair got sick of the love Romeo-wannabe Reivier, and revealed herself. She made Reivier think that Serai did exist, and she had just been taking her place. Lunair the Great Actress has been planning all of this out the whole time! Her mess in Gaurav town had all been planned beforehand, and Sheriff Douglas was a fellow conspirator! Mimi is the embodiment of evil and all evil! Rayne and Rayen were under the control of Lunair.

Lunair isn't an Aerian - She made Reivier help him intrude Aeria. All of this a plan to get "Birdie" back, because Lunair is paranoid! Yes, Lunair is Leah. Neither were Kardian and Kaire actually Aerians - was there any proof of their yellowhairedness in front of Reivier? Vikon is Lunair's father, the one who wants to stop her from meeting Birdie. Reivier is Goatie. Serai never died, because there is no Serai! The one who came back from Aeria wasn't Serai, it was Sheriff Douglas in disguise! Douglas was the thief. Huey never existed.

See, everything's falling into place now! Whoopie, now worship the large bombs.
 * This theory has been approved of by Reives himself. Woots.

The person Reivier wished he'd never met isn't Lunair.
Everybody automatically assumes that Reivier is talking about Lunair at the intro, since it's the most obvious (and logical) answer. They are wrong. Reives just tricked us all into believing this, hoping that no one notices the obvious twist. Dasha did fulfill Reivier's wish. We only have to find who was that woman he never met.

Okay, the chances of this being true approach zero, especially as the only important woman Reivier hasn't met is Lunair's mother (as far as I remember), but still, why not?

The introduction wasn't the first time Reivier had turned back time.
It wasn't the first time Reivier had turned back time. He's turned back time with Dasha's help for a thousand times. He just can't remember it consciously, but subconsciously the aftermath of all those memories have been eating him from the inside. We shall expect to see more deranged Removier.
 * Jossed. Thankfully; that'd be depressing.