There was a lot of things that they regretted about the early days as team Voltron.
Shiro regretted bringing kids into a war and then leaving them for those months that he was gone. He should've never put a burden on Keith that he never wanted and one that ultimately drove him away. He shouldn't have pushed Pidge too hard, shouldn't have taken Hunk's kind nature for granted, shouldn't have made Lance feel alone.
Keith regretted being so impulsive, and a little selfish. He knew that the team would never want him to leave but he did, he just had to get answers about his past and what that could mean about his future. He never wanted to be a burden on those around him and if he stayed he'd been just that. Keith regrets bringing Shiro back to space, back to the war that had already taken so much from him, and that he left him alone. He also shouldn't have made Pidge stay when all she wanted was to find her family, shouldn't have scolded Hunk for his jokes, shouldn't have taken his anger out on Lance.
Pidge regretted very little when it came to the early days, she worked hard and got results but she wished she was faster. She wished she could've saved Matt and her father from their traumas earlier. She wished she slept more sometimes too, and wished she had thanked Lance and Hunk more both for their time together at the Garrison and for making the transition into space easier. She should've helped them feel more comfortable like she did, she wished she had stopped Keith from leaving like he did for her, she wished she'd seen the cracks in Shiro's calm before he broke sometimes.
Hunk had many regrets the second they left Earth. He wished he had been able to better pull Lance in and keep him under control from his crazy antics, and from hurting himself. Hunk regretted a lot in the beginning but he grew to appreciate the things that his decision led him to, like saving planets and to the formation of team Voltron. But right now he regrets having so many regrets. He regrets getting lost in the tech with Pidge and taking some experiments too far (no one needed a walking pepper), he regretted joking too much about Galra Keith and the fact that they let him leave so easily. He regrets forgetting that Shiro was human and that he was allowed a break and to make mistakes, he regrets not helping his best friend feel needed and comfortable in the team and as his role as a best friend.
Allura and Coran also had regrets.
Allura regrets not being able to say a proper goodbye to her father, to her people, and to Altea one last time. She regrets pushing the paladins so hard in the beginning because once she was one she could see how stressful that must've been. She regrets not being able to stop many of Haggar's attacks because she was too slow or too afraid of her inability to perform her magic on the same level the witch had. She regrets having to rely on Coran so much to fill the shoes of a parent to her. She regrets not listening to Shiro as much as she should have and that she didn't do her best to get close to him and her paladins. There was so much she could've known about them sooner.
Coran regrets not being able to do more, both when Alfor was alive and Altea was under siege when they both needed him most. He regrets not pushing Alfor in the cryo pod instead of allowing him to push him instead. He regrets that Allura had to have that heavy burden placed on her at such a young age. He regrets not talking to Shiro more and not helping him through his trauma soldier to soldier, he knew what kinds of horrors that can do to an Altean mind he should've known it could do the same to a human mind. He regrets not calling the young ones by name more often and for allowing Lance to put himself in danger that first time when the castle's crystal exploded. He regrets not being able to shield them more effectively from the brutality that was Galra warfare.
They all had regrets.
Lance had them too, right now he regrets having so much valuable information in his head. He regrets that he had felt so uneasy in his role on the team that he just had to make himself useful and now he was an expert tactician and had more information about the coalition and its members, and he even managed to know more about the Galra and it's military than anyone else on board. He regrets making himself useful because when you become useful you get a target. Because now as he bled out in Keith's arms he had too much left to do. It didn't help that once the team found him they did their best to save him. But their best wouldn't be able to help him now it would be too late and take too long for him to heal.
"T-take me to the memory chamber" he hissed and the group halted on their way to the medbay and argued with him. "You need the information I found on that Galra command ship as soon as possible, a healing pod will take too much time," he said then assured them that he was fine and wasn't in critical condition.
Except he was, not only had he lost an incredible amount of blood but he was also sure the blade he'd been sliced open with had been laced with poison. He wasn't going to make it out of this one if he was going to bypass a healing pod. So yeah he would regret this decision in the end when his end came because he still had so much left to do.
He still needs to help Shiro through his nightmares, he needs to tell Pidge that he found her dad and actually talked to him, he needs to tell Hunk that the galra gun he was repurposing had to have the settings reversed for better usage and he needs to taste test the pie he was planning on making later. He needs to help Keith to feel comfortable on diplomatic missions, he needs to tell him how much it means that he came back.
He needs to tell Allura that the castle had over twenty volumes of ancient magic books she could study to feel more confident in her powers. He needs to tell Coran that there were also a lot of photo albums of the fifth company of the Altean Royal Guard and of him when he was a cadet with his first team.
He still needs to make it home to Varadero. To tell his mom that he was alright, that the Garrison were a bunch of liars and to not send Nico there because he thinks that there is a conspiracy there and he wanted to crack it, it's not worth it because Lance knows his brother would get into also sorts of trouble trying to prove it. He needs to get back to see Louisa's baby, his nephew.
He needs to tell Ricki that his music is the best and that he was always jealous that he was in a band and that they actually sounded good. He needs to tell Sara that he missed her loud wake-up calls every day he was in flight school because the alarm clocks at the Garrison were the worse. He needs to tell his dad that he was the best and that he did his best to be just as kind-hearted as he is.
He still had so much to do and he regrets not being able to do all the things on the list.
Getting his mind to open up to the castle to let all the information he has to flow into the tube next to the pod was easy. Lance smiled as he remembered something he had said the first time they had been in this part of the castle.
"When I go I want all the stuff in my brain stored in a giant ship"
"The amount of information in your brain could be stored in a paper airplane" Keith had teased effectively getting a rise out of him as he struggled to find a decent comeback through his embarrassment.
"Oh yeah well the amount of information in your brain is-" he failed.
"Well, it's less than what I have" Lance huffed crossing his arms to keep himself from doing anything else more embarrassing.
"Oh good one Lance!" Hunk the bestest of friends backed him up.
Lance almost laughed at the memory but began to cough up a lot more blood instead.
"Lance we need to get you out right now!" Hunk yelled but Lance shook his head and insisted that he was almost done. At this point, he was sure they could tell that Lance was more hurt than he had said he was.
He focused on the task at hand, the information first then he can upload his jokes into the castle mainframe. He's sure Pidge will have a fun time figuring out how to get his AI to stop popping up in places it doesn't belong. Because if Alfor's AI managed to hack the entire castle then he was sure his would be able to at least do some leisurely haunting as well.
It was surprisingly easy to give up all himself to his team and to this computer. Lance didn't want to disappear forever with all his regrets so he'll leave them here instead.
"All the stuff in my brain gotta get stored in this giant ass ship" Lance mumbled as they again tried to get him out of the memory chamber.
That scared them into action and the last thing Lance remembers is being embraced by his team as he slipped out of the pod. His eyes grew heavy, his mind felt oddly empty, and his body felt spent. Lance wondered if this is what dying really felt like because it really did feel like falling asleep.