Cardfight Vanguard/WMG

Aichi being jerk from Psyqualia was due to Kai being his role model
Psyqualia doesn't really turn everyone into a jerk like Ren, but exacerbates internalized feelings. Aichi acted the way he believes Kai acts, such as calling his teammates weak, and other acts of rudeness not too far off from Kai's actions in previous episodes. Made all the more obvious when Aichi invoked Kai's "Final Turn" flair. It just snowballs into a sense of cockyness that has taken on a life of its own.
 * And Ren has Psyqualia too, but who was Ren's friend? Kai.
 * Aichi and Ren's personalities grew more extreme when Kai pretty much dumped them.
 * Kai is a Yandere factory.

Aichi is the Big Bad and a Villain Protagonist.
As of Ride 40, we know three things; The theory is this; Aichi will become lost in his own powers just as Ren is now, but Ren will be saved and redeemed by Kai. As Aichi and Ren are parallel characters, Ren will finally succeed in getting Kai back--by going over to Kai's side--where Aichi will fail by deserting to become the leader of the team Kai used to serve. Aichi will still be the main character, but season 1 is his Start of Darkness and season 2 will focus on his defeat by Kai and Ren.
 * Ren has PSY Qualia/PSY Powers
 * Aichi has these same powers, activated by Ren's presence in Ride 23
 * Kai is concerned that Aichi is being overtaken by his powers, as Ren had been overtaken beforehand. From his brief flashback, we saw Ren collapsing into Kai, mirroring the way Aichi had into Misaki from overexertion.
 * As of Ride 41, this...actually seems to be happening.
 * The Big Bad status and Start of Darkness was jossed by Ride 50.

Cray is real and PSY Qualia allows its users to connect to it
There have been a few hints so far as to this. Firstly, the marketing fluff for the Vanguard Gloves states that the crystals on them were 'mined from the Planet Cray'. Secondly, whenever Aichi uses PSY Qualia, he seems to actually spiritually appear on Cray, more than 'image it', he actually seems to astral project himself onto Cray. And thirdly, Kai's line from Episode 40: 'you're the one who's doing the fighting'. The implication being that PSY Qualia actually allows something else to fight the game for you, like your Vanguard.
 * If Cray is real, PSY Qualia is then like an enhanced 'image it'. More than imagining Cray, you're actually there in astral form, which is how Aichi has had 'winning image' of his upcoming fights with Miwa and Team Fragrance. The 'possessed by the Vanguard' point also explains how Aichi put together his new deck at the Caesar Training Camp without being aware of it, he wasn't in control at the time, one of his Units was.
 * And, this could also explain something about why Ren went off the deep end. If PSY Qualia is basically Demonic Possession by the Vanguard, then who's to say that the Vanguard can't stay like that? Ren runs Shadow Paladins, which are canonically an Anti-Hero type faction. What if Ren used his PSY Qualia so much that in the end he couldn't come back, and this whole time 'Ren' has actually been a Shadow Paladin Unit maintaining control of a human body?
 * It seems more likely that PSY Qualia warps someone's personality. In Turn 41, Aichi's personality begins to match that of Ren, but he still has some actions that don't match. (Like his look of confusion when Crusher asks him if he took out the wrong card.) Also, "being possessed by the Vanguard" is the opposite of what Riding does according to the first two episodes. (Kai says that it involves possessing the unit that you Ride.) If they were being controlled by one of their units, it would make no sense for Aichi and Ren to act the same way when they don't play the same type of decks. (And their behavior should change when they change their Vanguards, which it clearly doesn't.)
 * It should also be noted that none of the Royal Paladins, and especially Units like Blaster Blade and King of Knights, Alfred, are in any way evil if the Character Development the Royal Paladins have had is to be believed. None of them would likely exhibit any of the evil personality traits that Aichi has shown in Episode 41, further discrediting the theory of the Vanguard possessing the player.

After the tournament is over, Morikawa will "power up" the structure deck Shin gave him by adding a bunch of Grade 3 units
... and in so doing bring his winning streak to a crashing halt
 * Confirmed

Aichi let Morikawa beat him in Episode 51.
The anime was careful not to show his hand and with his deck, it's unlikely that neither of the two cards remaining in it could block. In addition, he still had another Galatin to intercept, so even the weakest shield possible (5000) would have been sufficient. Combine that with the episode's lesson being that winning wasn't all that mattered, his shy smile and Morikawa's obvious joy at winning for once, it's likely that Aichi decided to let him enjoy it.
 * Well, I'm not denying your theory, but the anime actually did show Aichi's hand - it was Akane and Soul Saver Dragon. As a Grade 2 and a Grade 3, neither of them could've blocked.
 * This actually confirms the theory. Akane has a shield of 5000. Grade doesn't determine if a card can block or not, only Grade 3 can't block.

Aichi will integrate this card into his deck at some point.
Almost seems like a no-brainer; I mean, just look at its abilities. It's like the card was created specifically for the plot...Or maybe the plot for the card...Not to mention the plot seems to be having Aichi hold onto the Shadow Paladins as of episode 53. Considering it was announced just as the anime is heading into its climax arc...The timing is just too good. Plus, it would just be really cool.

Due to unexpected circumstances, Ren will fight Aichi instead of Kai
Aichi will use his PSY Qualia to cancel out Ren's and after he wins, Kai will acknowledge that Aichi has become stronger, leading into their long-awaited rematch which will finish out the series.
 * Kai has specifically stated that he wouldn't participate in the tournament and Aichi more or less said that he would overcome Ren. So the circumstances can't really be said to be 'unexpected'.

Ren will somehow incapacitate Kai after the upcoming fight Kai's bound to lose
He's bound to lose the fight anyhow, to add extra weight to the fight between Ren and Aichi. It would get Ren evil-points and Aichi would have even more of a drive to win the nationals. Furthermore, the episode previews mentioned how psyqualia 'locks people into an image of solitude'(very paraphrased), so my guess is that Kai is going to experience a more permanent version of the image psyqualia users show their opponents after they win a battle.
 * Like what Aichi did to Kyou in episode 42?

Ren and Kai will become so engrossed in the fight that PSY Qualia will cause a catastrophe and force a Draw
The fight is already played up as spectacular with a ring of fire encircling the field, and Ren used PSY Qualia to create a mass vision before they even began. It's not unlikely that as the fight continues Ren will become furious and ramp it up. Aichi's presence may begin to affect it as well, seeing as Ren was able to influence Aichi's before he had even discovered it. It seems that a twist like this would be a good way to lauch a second season and manage to keep both undefeated characters undefeated.

Aichi will be overtaken by PSY Qualia again.
Not necessarily during his eventual fight with Ren, but maybe some point in the second season. Aichi was only under its control for a (surprisingly) short time. After all, he still has the Shadow Palladin Deck. Who's to say he won't be tempted by the power again?
 * More likely is that he'll use the power without being corrupted. He told the Royal Paladins that he wasn't rejecting Psyqualia and fully accepted the responsibilities that came with it; he just wanted to become stronger by himself before using it again to avoid corruption.

The reason for Aichi's switch in Asia Circuit is because his old cards have too much emotional weight attached to them.
There's too much at stake with these decks--many of the cards are extremely important to somebody, and with all that pretense attached to them, Aichi would have no more room to grow because he wouldn't feel free to edit his deck or change tactics. Blaster Blade is attached to Kai and is his first card, Blaster  is attached to, Majesty will bring up unpleasant memories of season 1's final battle, and Alfred is attached to Suiko and his involvement with Ultra-Rare. By putting away his old decks and using the Gold Paladins instead, Aichi can get a fresh start and be free of being tied up by the past, which gives him more room to grow as a fighter.

Ren is unaffected by the memory change caused by Takuto.
The idea that Aichi is able to remember his previous deck while no one else does (outside of the brief flash Kai experienced), is because Aichi was a user of Psyqualia. Thus, Ren, who also used one of the Clans that were sealed and was also a Psyqualia user, can remember what things were like before. Also, like how Aichi is experiencing confusion from everyone believing that he's always used Gold Paladins, Ren has experienced this, but was able to adjust, thus being as skilled with the Gold Paladins as Kai is with Narukami, but able to recall the same things as Aichi.
 * And Ren was able to adjust so easily because of how spacey he is.