I absolutely love Saiki x Mera because it conforms more to what Saiki's motivations are shown as in his thoughts and actually addresses them, leading to a more complete version of relationships.
It still has its' flaws though, just like any other ship, but I think that it's actually more well done than anything else, and makes it real for me.
Throughout the series, Saiki communicates his impression of Mera using the way that he uses his thoughts. In fact, in chapter 47, he had no trouble with complimenting Mera.
Communication works well in a relationship, which isn't really that much prevalent in Saiki's hypothetical ships. Except for Aiura, that's a decent ship.
Mera also says that she hadn't talked to anybody about it, which is important for later.
And we also know that Saiki feels a bit bad for her in chapter 55 for her being poor and gives her a ticket anyway even if Teruhashi would come along to the trip.
Just like everybody else, Mera allows Saiki to have his red bean enmitsu. And since Saiki learned of Mera's gluttony throughout the trip, it means that Mera isn't just confined to her gluttonous character, as will be later seen.
And in chapter 68, Mera states that she would embarrass herself, which is a trait that Saiki also has which would make them understand each other well. And the fact that Saiki goes to Cafe Mami regularly would mean that they have more off-screen time.
In fact, Mera probably understands what it's like to be embarrassed by a role better than anyone else, even in the later parts.
And then, Mera's character peaks in chapter 108. While you have the travesty that is Saiki hanging out of a window to hide from Teruhashi, Mera invites Saiki to her job.
Even though it could shut down, it's the intention of Mera that shines here. Her intention was to give Saiki 100,000 yen, which is more like a business offer, even though it would lead to less free time.
And Saiki responds to her by "kindly asking her to leave" which is contrasted from all the other interactions in the same chapter. I am willing to bet that it's because of the unusual positive type of bond that they have.
And Mera was the only one to deliberately open up to Saiki, instead of being revealed by telepathy, and Saiki kinda accepts it, which is another sign that a relationship might work out between them.
And then there's chapter 131, and chapter 151-155, where Mera eats too much, maybe expecting a rescue ship. But even if she didn't go savage, she might have eaten all of the food anyway because she starves herself for her siblings who are seen in chapter 55.
She works 12 jobs. These chapters 131 and 155 show how Mera's gluttony is flawed for Saiki in some way, but I genuinely think that Saiki thinks that she's made up for it in chapter 240, much like how all functional relationships need some balance in some way.
So then Saiki wants to help her, and he admits in chapter 187 his concern for those chapters.
And when the people admit that they would use Saiki as a hostage against her, which causes him to completely humiliate them. And Saiki has been shown to be very resistant to anger even while listening to very vile people, so it shows that he's even more protective of Mera.
In chapter 202 however, Saiki doesn't approve of Mera's way of doing things, but I guess he understands. She mentions that she has 6 jobs instead of 12, which is improvement.
And from thereon, Mera isn't shown eating a lot, acting on her gluttonous nature.
But chapter 215 is really special because Mera actually does a task that Saiki wants because he asked her to, and she did it flawlessly.
And what does Saiki do? He smiles at her. This is one of the chances that Saiki had for opening up with someone, other than chapter 279 with Aiura and chapter 280 with Nendou.
Next is chapter 217
This statement by Saiki is him being serious. There's also another moment where Saiki doesn't trust Mera with a pet earlier, which is fine.
Chapter 240 starts with Saiki concerned with Mera.
Next, Saiki mentions that Mera visited him on his birthday, and he isn't made uncomfortable by it, which is leagues better than when Teruhashi visited Saiki.
This moment speaks volumes about how Saiki trusts Mera in this way. And then Saiki helps Mera, because the cheese fountain ends up behind some classmate and she fell forwards, tripping over her bag and landing onto her lap which makes no sense.
And the reason why Saiki helped her was revealed earlier, it was due to Mera thinking about her family, especially after Saiki thinking that he had no pity for her.
Later when Mera becomes rich due to bitcoin, Saiki asks who she is.
But I think that Mera has genuinely completed her character growth by going to Oshimai in chapter 277 by hitchhiking.
The 4koma then shows her not failing at her job anymore
Saiki x Mera is perfectly balanced with no exploits whatsoever. It's like something you would see from Ugly Betty; a show that does relationships very well. It would be a par for the course ship, like 7/10, but since Saiki x Mera is in anime territory, it's ignored.
And even though Saiki mentions some problems, he has always known that cared for Mera in some way. The statements back it up. The tone backs it up. And to some extent, I think that Mera is to some extent, what Saiki could need, due to his guilt for his psychic powers.
That's the beauty in Saiki x Mera, because Saiki's thoughts are serious nearly all the time, so the fact that everything matches up here is just a chef's kiss. 9/10.
Edit: Friends becoming lovers due to friendships are very common and reasonable in real life. Google search says 70%. So if both of them feel like they're mature enough and ready to deal with each other, then they could make it work.