They can be canon by falling in homo for each other, obv. Okay but seriously:
In a completely hypothetical, what-if scenario, I can see Ereri being canon in a slow, gradual manner that would take years and years to build. It’d be very subtle, so subtle to the point that neither would even notice it. Maybe they’d spend their whole lives never realizing they were in love. Maybe something would happen that would make them realize they loved the other. Or maybe they’d know, but they’d be the type to never say it out loud because things are perfect as they are and things like saying I love you is of little consequence to them; means very little to them when surviving is the most important thing they can do in the world they live in.
I can see them being canon as Eren grows up and matures. The only close thing to “love” that he knows is his loyalty to Corporal Levi. He grew up idealizing this man, and being close to him and serving beneath Levi is all Eren desires.
I can see them being canon as Levi continues to grow fond of Eren. At first it’s just the fondness in the way a superior officer is fond of their subordinates; then it grows more into a I’ll-never-let-this-brat-die kind of way. Days and weeks and months and years go by and they’ve fought and survived so many fights against the titans. Eren’s grown to be more than just a subordinate and a tool for humanity’s survival. He’s grown to be Levi’s right hand man, the only person he can trust, the only person who knows how Levi likes his tea, how Levi likes their home to be clean, how Levi likes the laundry to be done, his food to be cooked, his hair to be cut and his clothes to be folded. He knows all of the little mundane things that his Corporal prefers, and before Levi knows it, he can’t survive without Eren Jaeger.
And in a completely hypothetical, what-if scenario where the titans are wiped out and Levi and Eren live on, I can see them growing old together. Eren still continues to take care of Levi. They don’t know if it’s love per se — neither really know what love means — Levi just knows he can’t stand anyone else to make his tea and Eren can’t stand serving anyone else other than Levi.