If it’s possible to make Johnlock canon by having it happen off-screen, then Johnlock is already canon, because I am pretty sure it’s impossible to cram more gay into this show than they have already without including make-outs.
However, we are still arguing about whether Johnlock will ever become canon.
Which indicates that the only way to make Johnlock canon is if it happens on-screen.
Alas, I don’t think we’re ever going to get that. I think that right now Johnlock is as canon as it’s ever likely to get.
The long answer:
I should note that I have a much more positive read on S3 than most people do. I don’t think Sherlock was pining; I think that by the end of this season he was feeling pretty happy and fulfilled in terms of where he stands with John. And I think John’s been perfectly well aware of his own attraction to and feelings for Sherlock for quite some time, to the point that before Sherlock faked his death, I think John was feeling fairly committed and settled down with him.
In HLV, I read that conversation about nothing on the tarmac as the two of them not NEEDING to say anything because at this point they have a full, mutual understanding of what’s between them. When you both know how much is between you, trying to put it into words is a waste of time—and that is exactly what they both say to each other, right off the bat.
So you know. That seems fairly canon to me.
But here’s the thing.
Clearly, that offscreen/onscreen question is all-important, when it comes to depicting a queer relationship in entertainment media. Because even those of us who WANT Johnlock to become canon are arguing over whether it has and whether it ever will. Because this is how it works with queer relationships in entertainment media. We have built up a history where the default assumption is that they aren’t happening. So unless/until 1: a kiss happens on-screen, or 2: somebody baldly, unequivocally states their sexuality and relationship status on-screen, the status is “assume it isn’t happening, unless by some chance you prefer to think it is or you happen to belong to the sub-culture that recognizes what it means to carry a colored handkerchief in your back pocket.” It’s in the closet, essentially.
So like I said, the first half of my answer to your question is that it HAS to happen on-screen if they want to do away with the veiled “are they/aren’t they” nature of their relationship.
Which incidentally is REALLY FRUSTRATING when it comes to this show, because LOOK AT IT. Short of hanging banners that read “CONGRATULATIONS ON YOUR NEW GAY RELATIONSHIP,” they couldn’t be more canon if they put a ring on it. Sherlock said his damn wedding vows! (John not needing to, seeing as how he already took the hard road through “better or worse till death do us part.”) Everything about John—from his narrative arc to where he’s placed and how he’s dressed in any given scene—falls absolutely in line with the traditional narrative arc of the romantic interest.
So the second half of my answer to your question is that aggravatingly, I think that it ISN’T ever going to happen any more explicitly, more incontrovertibly, more on-screen than it has already.
The writers and actors have all been very clear that they want to leave Sherlock and John’s relationship and sexuality ‘open to interpretation.’ And it’s true that this group does indeed lie gleefully to obfuscate upcoming plot twists. But on this particular issue, I personally cannot see my way past the weight of that history full of heteronormative entertainment media—and Moffat’s apparent desire to be able to fanboy-identify with his own heroes (this is a man who can invent Captain Jack Harkness, but who automatically assumes that if Holmes finds Irene Adler interesting it’s because he wants to bang her)—to doubt that they mean what they say.
Now I think that does mean they they are equally committed to not closing the door on the possibility that Sherlock and John do want more or are more to each other. But I think that everything that happens between them will continue to happen either off-screen or ambiguously, in a way that people who don’t want to read romantic interest between them will have the ability to dismiss.
It kind of makes me want to kick things. And I’m still hoping I’m wrong. Aside from the fact that I’ve spent over half my life wanting somebody to write an explicitly queer Holmes/Watson relationship, it’s damn important to a whole lot of people that queer relationships not continue to be treated like their proper place is “in the closet” on a societal level. And it’s frustrating—not to mention insulting!—to have a show that skirts the line this closely and refuses to just come out and say it.