Thinking about an AU where Ritsu is the one who ends up being Reigen's student instead. Say he's there when Mob sees the sign, dissuades him from going in because "it's probably a scam anyway!" But then of course, little envious Ritsu ends up coming back the next day, unable to help himself at the chance, the possibility that someone could help him develop psychic powers like his brother. He figures Reigen out almost immediately, of course, because he demands to be shown Reigen's power. But Ritsu is desperate, and Reigen has always had a kind way with words, and soon enough Ritsu and Reigen are going ghosts adventures! Leaving Mob behind :') Even more alone than before :'))
And as S&S fills up with things of ACTUAL spiritual significance, like artifacts and purified salt and herbs at Ritsu's obsessive insistence as he tries to find an in to psychic powers and Reigen tries to encourage him without encouraging him, Ritsu and Reigen get closer. Ritsu is more surly than Mob, less inherently trusting and more emotional, more easily annoyed—but he has no reason to hate Reigen in this verse. Reigen has never even met Mob, much less used his powers for his own gain. Reigen grows incredibly fond of this desperate boy, good at everything but the one thing he wants to be good at, and tries to lead him away from those feelings of jealousy and inadequacy. In turn, Ritsu himself is healthier. A little easier going, though still almost perpetually bristling with temper 😮💨
He and Mob are closer as a result! He becomes Mob's only friend, more open with his own thoughts and feelings and, thanks to Reigen, far less afraid of him. Mob is more emotionally stunted than ever, and though he has some tentative acquaintances in the Telepathy club, he's still mostly alone and isolated. All of Reigen's helpful advice on dealing with his autism (identified by how Ritsu speaks about his Shige-nii and the behaviors he says he exhibits) is relayed through Ritsu, who comes to understand Mob a little more because of it.
But—oh, didn't I mention? This is a soulmate AU :)
Ritsu has two marks; one for Mob, and one for Reigen. Mob never develops a mark for Ritsu, but he expects that, obviously. Reigen also doesn't develop a mark for Ritsu, but given their age difference, Ritsu also expects that. He's a smart boy, and he knows that not all soulmarks are reciprocated. It's fine. They don't need to be. Ritsu has mostly made peace with his lot in life.
That is, before Mob and Reigen finally meet in person. And instantly develop matching soulmarks.
...Ritsu's psychic awakening is only slightly better than his brother's.