Not sure if you're still active but do you happen to know any fic featuring an older time traveling Keith and a younger Shiro? I have a hankering :3
Yes, there's a bunch!
Equalize - stardropdream
11k. Teen.
Shiro watches as Keith (Keith, different, with long hair in a braid, broader shoulders, and older face) looks around the room wildly, trying to place where he is. Then he closes his eyes and laughs, a soft, disbelieving chuckle. Shiro watches the tension ease from Keith’s shoulders. “Right,” Keith whispers. “I remember this now.” Or: Keith from the future ends up in the present. Shiro copes the only way he knows how-- by squashing all his feelings back down again.
you can always find me here (the coming home remix) - perfectlyrose
7k. General.
Keith continues to watch himself fade and flicker. He’s staring straight at Pidge’s terrified face watching her form words that he can’t hear when he blinks out of existence. There’s a long moment of darkness, endless and complete, before he fades back into physical existence. He blinks into the dim light and recognizes the dull metal walls and purple glow in an instant, stomach sinking to his feet. Just his luck that the failed wormhole transported him straight into a cell on a Galra cruiser. A quintessence experiment gone wrong sends Keith skipping backwards through time, back to a Shiro he hasn't seen in years, and one he's never seen. Keith just wants to get back to his Shiro and his time.
Up and Down Memory Lane - SaltiSnacks
8k. Mature. Contains: Shiro/Curtis, Keith/OMC
A time-traveling Keith is on board the Atlas. Shiro overhears a conversation. The house of cards crumbles.
Yesterday, We Begin - arcadenemesis
13k. Explicit.
"Where did you come from?" Keith huffs a laugh. "A long, long way from here," he says, plucking the cherry from his glass. Shiro's eyes follow it all the way to his lips, and Keith watches his throat bob when he bites down and plucks the stem. It's an evasive answer, but if Shiro had any follow-up questions, Keith thinks they're long gone. "Do you want that?" he asks when the silence drags, gesturing at Shiro's cherry. It seems to snap him out of his trance. "N-no! You can take it," Shiro stammers, pushing the glass toward him. "Please."
A mission takes Keith back in time, and leads him straight to Shiro.
Stars Fading - zjofierose
2k. Teen.
Keith knows where to go because Shiro had told him, knows the little house by sight from the many pictures Shiro would scroll through when he was feeling melancholy or nostalgic. He’s been waiting for this chance his whole life, wondering when the moment would come - the second his ship breaks orbit above Earth while simultaneously displaying a date in the past, Keith knows: it’s time.
twenty-thousand leagues above the ocean - ErinNovelist
8k. Teen.
“Please,” he begs, glancing up at Keith again, heart throbbing against his ribs. Bare bones are the only thing stopping him from throwing himself at the man he’s known forever. “Just tell me who you are.” Keith looks up at the silver sky, at the worn jacket Shiro clenches close, at the shaky hands shoved into deep pockets to keep warm. “My name is Keith, and I know that doesn’t mean anything to you right now, but someday you’re going to be someone very important to me.” He sighs and tightens his grip on Shiro’s shoulder, leather soft beneath his fingertips. “You already are, and you’re the only reason I’m not lost right now.” “I don’t understand,” Shiro tells him because he doesn’t know what else to say. Keith only cocks his head and smiles. “I’m lost in time, Shiro. And… well, you’ve always been my guiding light, I guess.”
Life We Chose - stardropdream
3k. General.
Not too far out from the Kerberos launch, Shiro finds an unexpected guest waiting outside his quarters, both familiar and unfamiliar.
Stuck in Gravity - stardropdream
11k. Explicit. Contains: Keith/Keith/Shiro
Keith thinks that everyone should be a little more concerned about this supposed "future" him. He's just not buying that any version of himself could be so confident, so happy... so clearly loved. (Or: the "I had to go back in time to teach my younger self that he's worthy of love" time-travel threesome.)
Only an Echo - stardropdream
10k. Explicit. Contains: Shiro/Shiro/Keith/Keith
Shiro should have guessed that things were about to get weird when a wormhole opens up in his bedroom and out step the future versions of Shiro and Keith. “Right,” the older Keith says, smiling a little. “Well. There’s no easy way to explain it.” “We’re—” the other Shiro begins. “— here to fuck you,” the other Keith cuts off. “Not quite how I’d phrase it, baby.”
Or: Recently married Shiro and Keith decide that the best wedding present they can give each other is "let's go back in time and fuck our past selves." So they do.