Here are my ideas on which characters would pair up from the series Attack on Titan and Voltron Legendary Defender.
1.Takashi Shirogane and Commander Erwin Smith
These two would pair perfectly together because they are both leaders and they have both been through a lot of crap in their lifetime, not to mention, if anybody has not yet pointed this out yet, they both have had their right arm chopped off, so they can both relate to each other.
2. Keith Kogane and Captain Levi Ackerman
These two would go good together because they are both willing to follow their superior's lead even when they think otherwise. They are also leaders themselves, when said Shiro and Erwin can't be in command, Keith and Levi would step up to the plate. They also have a sort of a crappy past and have incredible skills, with Levi having "Ackerman Blood" and Keith being "half Galra".
3. Lance McClain and Eren Yeager
They both are paired well in my mind, because they both have awesome potential. Lance was able to see that the Shiro that was currently with the Paladins, was NOT the real Shiro, rather, a clone Shiro made by the wretched Galra, and Eren, having the power of the Attack Titan and being able to control all the mindless titans nearby.
4. Pidge Gunderson and Hange Zoe
Both have a very deep interest in science and figuring out what the world is about and just genuinely exploring the outside world. But the difference between these two is that they live in two completely different era's. They both get excited when they discover something new and are willing to go the extra mile to figure out what exactly makes it work.
5. Hunk Garret and Nanaba
These two I feel would be perfect together because they are both very sweet and kind, while they are also very brave and strong when it comes to battles. They are both very loyal to they're friends and are absolutely loveable as well.
6. Princess Allura and Historia Reiss
These two beautiful and powerful women would be the perfect pair because they are both willing to fight for what's right. They both have a feiry personality and they are both beautiful in general. They also have awesome skills and will kick your ass if you underestimate them.
7. Coran and Mike Zacharias
I figured I mine as well throw in this pair of characters, just for funny purposes. Anyway, these two would be great together because they're both weird, but in they're own special kind of way. While Coran will ramble on and on and on, Mike will simply sniff you like a dog that's just getting to know you. But hey, that's alright, that's just them.
8. Prince Lotor and Reiner Braun
I picked these two to be compared because they are both conflicted with themselves. While Prince Lotor is hellbent on harvesting Altean quintessence for the future of the Alteans, Reiner is struggling with a split personality disorder.
And that's my take on the Attack on Titan and Voltron Legendary Defender Character Crossover.