What's Cold Without Heat?
While the return of Captain Cold would make a lot of fans happy, it's important to remember that the Leonard who is currently on Earth-1 isn't Captain Cold. Citizen Cold is a very different person, and throughout his appearance in Crisis on Earth X, he surprised his teammates with his touchy-feely side; especially Mick. Leo encourages Mick to talk about his feelings, tells him that he doesn't need beer, reminds him that 'due process is a thing' and he can't just shoot people left, right, and center. He even scoffs at the things that his Earth-1 counterpart used to say, such as the wonderful 'throw the plan away' line. He responds that that is ridiculous, that he always has a plan. In short, these two may look alike, but they are extremely different people.
This is going to make life very interesting for the next few episodes, and we can't wait to see how he interacts with the team. However, it means that even if Miller wanted to stick around for the long term, it simply wouldn't be the same. Doomworld Snart, Earth-X Snart, none of these guys are going to fill the hole left by the original (and the best) Captain Cold. Which means that it isn't necessarily a bad thing that Citizen Cold isn't going to be sticking around for the long term. It's better to give him a great exit this season than to see a poor imitation of the snarky Snart around for seasons to come.
Citizen Cold Sticks Around... For Now
So Leo Snart is going to be part of Legends of Tomorrow for another few episodes at least; why did he decide to stay, and how will he likely make his exit? There's a reason that he's sticking around, and given that Earth-X is now left without Nazi leaders it will be chaos; and every hero will be needed on deck. So why is this uber-do-gooder not headed home in order to help The Ray fix things? We're guessing that there is someone (or a doppelganger of someone) that Leo has some business with. That might well be Mick Rory himself, although from Heatwave's recent reactions to both Leo and Leo's stories of his own Earth-X counterpart, that may not be the case. Another possibility, of course, is Leonard Snart's sister, the Golden Glider.
The last we saw of Lisa Snart (Peyton List), she was still alive and roaring around Central City on her bike, and hasn't been seen in the Arrowverse for a while. On Earth-X, she may well be dead, and Leo might be staying on Earth-1 to try and find her, and even bring her home with him. He could also just be trying to recruit other heroes to help him on Earth-X (let's be honest, they've got a big fight ahead of them), or it could be the time-travelling thing that is intriguing him. Everyone who discovers the Waverider has someone that they want to go back and save; could the upright Citizen Cold be hoping to use it on his own world? Whatever his reasons for staying put, it seems that he'll have an episode or two more before heading back... or making another final exit. Given that he's got someone waiting, and fans have already had to see him die once, we're hoping for a simple return home, even if we'll miss him when he's gone.
Legends of Tomorrow will return in early 2018 after the midseason break.