Based on the prompt by jordanalane based on that popular headcanon floating around tumbler:
"Shiro asking Keith to marry him, but in the moment it's Keith who asks first."
When it comes to the big things in life, Shiro is a planner. He can be spontaneous, and often enjoys doing so with the small daily things. The number of times he's boxed Keith in against one of the Castle windows just because his boyfriend looks spectacular surrounded by stars, more than proves his willingness to act on passing whims, but planning is his security blanket. He finds comfort in having the path to his goals plotted and laid out before he even takes the first step. He's planned for nearly every big event in his life: Gaining admission to the Garrison, becoming a pilot on a major mission, escaping the Galra, and now as the leader of Voltron, he plans every fight and training regime he can. So really, planning and obsessing over every detail of asking Keith to make their implicit promise to spend the rest of their lives together formal, should surprise absolutely no one.
Doesn't mean that some people don't take the opportunity to tease him relentlessly for his behavior. He's in the middle of a war zone trillions of miles from earth. He can't just pop down to the local jeweler for a ring, and he'd be remiss in his duties if he didn't warn the others what he was up to, after promising not to tell Keith of course. Pidge and Lance in particular are tickled pink at his nervous over preparation. Lance keeps telling him that the 'mullet head' would be happy with a Go Pop ring as long as Shiro was the one who gave it to him, and Pidge insists on taking pictures chronicling him freaking out for posterity. She says she's going to give a photo album of the pictures to Keith as a wedding gift. He really hopes there will be a wedding.
Allura, Hunk, and Coran are infinitely more helpful. Once he finishes congratulating Shiro on his young love and reminiscing on the engagement rituals of Altea, Coran directs him to the Castle machines he'll need to create a ring. The old man even drags out a couple books from the library and helps him design it. Coran may have rather terrible tastes in food, but he isn't half bad at helping him make something Keith might actually wear. They create a metal alloy ring that looks a lot like silver to him, but Coran informs him the metal will stand up to anything short of a laser blast. Then, they inset three tiny purple gems, donated to the cause by Allura, that remind him of Keith's eyes. The final product is beautiful, and Keith could throw a punch in it without damaging anything but his opponent's face. He'll like that.
Hunk helps him piece together a close equivalent to the hover bike Keith and he had owned back at the Garrison. He wasn't a half bad mechanic, but he didn't hold a candle to the engineer's skill with alien technology. He'd have blown himself up ten times over if he'd tried to assemble it on his own. Hunk also gave him a shoulder to hyperventilate on and reigned in Lance and Pidge's teasing when everything got to be a bit much. The Yellow Paladin didn't always get enough credit for his willingness to be there for people. Shiro needed to get him something as a thank you gift when this was all over.
Allura was critical in timing and picking a location. He'd had an idea for how he wanted to propose to Keith since before he knew anything about Voltron or the Galra. Finding a place that matched the spot in head, without flying back to Earth, took a lot of knowledge about the galaxy and how to search the Castle archives. Allura came through, though. She found a planet fitting his criteria outside of Zarkon's influence, and just conveniently happened to open a wormhole to the system it was in that coincided with one of their scheduled rest days.
Now, all that was left was for Shiro to actually put his plan into action and propose. God help him.
...
The rapidly cooling wind whipped around Shiro, as Keith stayed a warm presence pressed flush against back, distractingly wandering hands wrapped tight around his middle. Shiro swerved slightly from his predetermined path to use a perfectly angled dune to launch them into the air, just so he could hear Keith's delighted laugh over the comms. The desert they where in could have been a twin for the one surrounding their shack back on earth, and they'd been out here for hours. Taking turns driving and riding their new and improved hover bike. Pulling dangerous stunts for thrills, racing from point A to B and back again as fast as they could, generally goofing off and acting like giggling idiots. He couldn't think of a place he'd rather be, or a person he'd rather be with.
They'd been having more fun today, than they'd both had had for a long while, but time was running short. The sky was starting to darken, and they had somewhere to be. There was a particular outcropping nearby, that stuck out above the rest of the land and gave a view for miles. That's where he had decided to ask the question. With the setting sun painting the sky shades of red and orange, and a day of joy behind them.
He needed to hurry or he'd miss his chance, though. He'd let himself be distracted trying to prove he could make a jump between the tops of two dunes. Keith had been teasing him about being too rusty to pull it off. He'd made the gap after about five tries and accepted his victory kiss in good grace. If he hadn't had a plan, he might of asked right there, but he was going to do this right. Keith was special to him. He deserved a perfect proposal.
The outcropping came into sight, and they still had some time to spare. He could feel the ring secure in a box in his pocket. Everything was going according to plan.
Shiro slid them to a stop in front of the path leading to the top of the stone. He tried to moderate his exuberance, before pulling his helmet off and climbing down from the bike. He'd never live it down if he gave everything away by arousing Keith's suspicions with his excitement. He turned to give his boyfriend a hand down, but Keith wasn't looking at him. Instead he was staring at him own helmet, twisting it this way and that in his hands. He didn't look happy.
"Is something wrong?" Shiro asked. He'd thought today had been a good day. Had he overlooked something? Upset Keith in some way?
Keith gave him a surprised smile. "No, no, everything is fine," Keith said quickly, "I was just thinking about something." He grabbed Shiro's hand and hopped down, giving him a peck on the lips as soon as he was steady, "Don't worry about it. It was nothing."
"Are you sure?" His own desire, to get to call Keith his husband, didn't override making sure Keith was alright right then.
"I'm sure," Keith said, giving him a reassuring smile before continuing, "You wanted to show me something right?"
Shiro looked at him for a moment more. He did look better now. He really must have just been thinking of something else. Not like Shiro could judge someone else for having intrusive dark thoughts. "Yeah, it's just up here," He said.
The view was just as amazing as he thought it would be. Keith's gasp of appreciation told him he'd made the right choice. Shiro gripped the box in his pocket hard. Everything was ready, he just had to ask. He could do this. "Shiro, I…" Keith's voice broke into his thoughts, before he worked up the nerve. He turned to where Keith was standing. The same unhappy look from the bike was back on his face, the only difference was he was looking at his feet instead of his helmet this time.
"Keith," Shiro said softly. Now he was worried. He reached out for Keith, tried to pull him into a hug, but he shook his head.
Keith took a deep breath, "I wanted to…You know I…" He started and stopped again, then seemed to give up on making a full sentence. Keith shoved his hand out, thunking Shiro in the chest, "Here," he said, looking anywhere but Shiro's eyes, cheeks beginning to turn pink.
Shiro looked down, and his thoughts immediately ground to a stop. In Keith's outstretched palm was a single ring. It was gold, with a single grey gem, and it was his size. "This is." Shock stole the rest of his words.
Keith had found his, though, "WillYouMarryMePleaseILoveYou," He said in a rush, finally looking at Shiro with equal parts hope and fear. His expression quickly tilted more towards fear, as Shiro didn't immediately get a response out. "Is it the ring? I can get you another one. Pidge and Lance helped, and they have terr-" Shiro kissed him.
Shiro kept kissing him as he took the golden ring from Keith's hand and replaced it with the opened box containing his own. Then he broke the kiss, "I'll marry you, if you'll marry me." He said.
Keith looked at the new ring in his hand, then to Shiro, then back to the ring, "What?" He said dumbly.
Shiro bumped their foreheads together, "I think some of our friends have been having a laugh at our expense." He explained with a smile.
Keith processed that, then his hand tightened around the box, "I'm going to kill Lance," He said, with sudden determination. Shiro kissed him again and again and again.
"Kill him after?" He asked between kisses.
"After," Keith agreed, throwing his arms around Shiro's shoulders.
Somehow, despite how much events deviated from his plan, the proposal was still perfect.
End Note:
Lance doesn't get murdered. Mainly because Keith is too busy being on cloud nine enjoying the benefits of Shiro accepting his proposal. Pidge teases both members of the happy couple that the honeymoon is supposed to happen after the wedding not the engagement.