Taking in fandoms one at a time.
Am a Resident Evil nut though.
First of all, welcome to the Resident Evil fandom! we’re happy to have you here in our little community. Feel free to post your opinions and whatnot.
Okay then, going straight to the point.
As a shipper of Ada and Leon, I think it is a definitive conclusion that their relationship is not one for the ordinary definition of a ‘relationship’—not one for the lovey-dovey type who later on will get married, settle down, have kids, be grandparents to the kids of said kids, etc—NO, they’re not like that, and probably will never be (though I love fanfictions of that—and who knows. Capcom is one crazy company). What we see instead is this fiery cat-and-mouse chase that puts the spectators on the edge of their seats—one second they would kiss, the next they’d be pointing their guns at each other. Theirs is this strong (and rather sentimental) bond that was forged on the crucible of death, danger and adrenaline. But still a bond, nevertheless—still an obvious attraction and attachment to one another (and Capcom can’t get more obvious than what their games portray). You may then point out that these emotions were only brought about by the heightened emotional attachment you’d normally have with your companion when faced with common danger, but the fact is that it endured even as this “common danger” passed; years and years after their first meeting and the chemistry is still there, the flirtations and pointing-guns-at-each-other and all that are obviously still all there.
So to (finally) answer your post, I don’t think their relationship works on the kind of ‘relationship’ expected of normal relationships—theirs is this sort-of touch-and-go relationship that is definitely not for the faint-hearted. Danger is normalcy to their relationship vocab; pointing guns at each other is their meaning of direct intimacy. So yes it would work, but only to their (Ada and Leon’s) definition of relationship.
And if the meaning of their relationship is seen on such perspective, don’t they already have that “long-lasting” relationship? I mean how many years has it been since the Raccoon city incident?
(But I’m pretty sure these two have had their moments of private intimacy—it was made pretty obvious with some of their dialogue in Damnation.)
p.s. forgive the lanky response—it’s what my mind can impulsively articulate within the given notice of time. I’m sure my fellow Aeon fans could articulate better responses.