Honestly, I think it's inevitable that they'll have to do some expanding on his character either way, whether mains and fans like it or not. Because the "masked serial killer with a dark and mysterious past" trope can only work for so long before becoming incredibly formulaic and stale over an extensive period of time. They work well as one-time characters in standalone movies, games, TV show seasons and novels - but when it comes to large, "legacy-franchise" IPs that are all about the corporate exploitation of familiar mascot-characters for more & more movie sequels, future TV seasons, game spinoffs, merchandising and etc? No. They lose their luster rather quickly and devolve into forgettable, redundant flanderizations of themselves.
And this is what Riot is aiming to do with their League IP. Go with a mixed MCU/Nintendo/Blizzard route and slap League's characters and lore into various new games & entertainment mediums, and see what works from there. That's the reason why we're getting an upcoming animated TV series about Jinx's childhood, and currently, a Marvel comic focused entirely around Zed. Both of these very popular champions were once in similar, murky narrative state like Jhin is right now. Both were unpredictable, sociopathic, chaotic-evil, villainous characters with vague motives and literal "fill in the blanks yourself" excuses for backstories. Hell, there were even people debating whether or not Zed was human before he received his recent lore expansion/retcon. That's how little we knew about them... But - that's no longer the case for either of them anymore. So it's very likely that a very popular champion like Jhin will receive similar treatment in due time.
However, I don't think they'll ever reveal his ENTIRE backstory because - as you and others have said - completely fleshing out his past, his characterization and his motivations would also ruin that weird sense of mystery that generally comes with serial killer archetypes who waver between between humanity and inhumanity. So removing ALL sense of mystery would completely destroy one of the most appealing aspects of his character.
So long story short - yeah, I think we'll definitely learn more about him. It's inevitable, and I wouldn't even be surprised if we manage to learn a few new key details about past in the last 2 issues of the Zed comic. But I _don't _ think we'll ever learn learn EVERYTHING about him (at least not for a loooong while.) We'll probably get occasional glimpses into abstract fragments or bits of his past that we'll be encouraged to "piece together" for ourselves or something like that, if Riot Odin's hints and comments are anything to go by. But who knows, maybe my assumptions are completely fucking wrong.
