This discussion may contain spoilers!
Seeing as I have an obsession with Clementine as a character and I'm extremely bored, I'd figure I'd preach to everyone as to why I believe her story needs to continue.
I'll start by extracting a few sentences from this article that details Clem's creation and Telltale's initial plans for the game:
"Though Lee Everett is the character you control in The Walking Dead, he wasn't the character that the game is built around. 'Clementine was literally the first idea,' says Vanaman, who planned Episode Fives’s outcome before writing a single line of dialogue."
"Clementine isn't just a collection of polygons on a television screen. She's broken through the barrier, securing a place in the hearts of many - a feat most video game characters never accomplish."
Those two quotes essentially sum up the message I'm trying to convey here. Clementine to me and many others is the very foundation of The Walking Dead Game series... Without her, I'd most likely lose a large chunk of interest in future seasons simply because she'd been developed for so long only to be tossed aside when things were about to get interesting.
Depending on the player's ending, Clem's reached an extremely intriguing emotional point. No matter what, she's more or less lost everything important to her bar Kenny/Jane if you let one of them survive. Hell, if you rejected the family during the Jane ending then she's even sacrificed her innocence; I want to see the apocalypse continue to affect her now she's approaching her teenage years, because those years are what will permanently shape her personality as an adult. Will she become cold or remain invested in the ways of empathy and humanity, rather than purely focusing on pragmatism?
Regardless of what ending you personally accepted, AJ is within Clementine's care. Having been raised by Lee and taught how to survive, I'm anxious to see how Clem will approach bringing up Alvin Jr, and this is something that could be explored within the third season. Perhaps you'll end up abandoning him, realizing that he's nothing but walker bait... Or maybe you'll do whatever you can to ensure this baby/toddler survives, considering him a beacon of hope and a living manifestation of the future? This would continue to build upon the theme of choosing between humanity or survival, serving as an excellent way of developing her character.
An argument I often see against Clementine's return is that playing as a child is boring and unsatisfying. If you truly believe that, then I'll direct you to one of the plenty of games that allow you to exercise mad power fantasies as a strong adult. Playing as a child is anything but boring. It's a concept that's been explored by barely any game developers, and seeing a young-un adapt to an apocalyptic situation is unique and immensely engrossing. Rather than simply ploughing through walkers with sheer brute force, Clem has to use her wits and intelligence to overcome physical threats, and that's something I enjoy experiencing a lot as a player.
Some people claim that Season 2's finale gave excellent closure and allows our imaginations to craft Clementine's future... But I'd disagree. Each ending is far too open-ended for the series to end here. I'd almost feel cheated if it did. These two seasons of development would be wasted, and there'd be no hook to Lee or any of the characters we originally came to know and love. The great thing about Clem is she's been with us since the very beginning, and that means we get the opportunity to hear her talk about old friends and previous events which overall make everything feel that much more real and believable.
What's the point of seeping back down to the drawing board and starting from scratch? It'd feel like a spin-off game if all previous development was rendered irrelevant and Telltale instead started up a new story. I'd rather Clementine die in Season 2 and that be it over with than get thrown into an entirely different group that we all know's going to end up dead anyway. Attachment would be extremely difficult without Clem being the anchor holding it all in place.
Maybe I'm cruel... But the idea of Clementine living a happy, normal life within Wellington seems too happy and far-fetched for The Walking Dead universe. I want to see her suffer! I want to continue her journey and I want to see the implications her experiences have on the people around her and her mental state. It's crazy how much potential Clem has, and I think it's more than obvious that her continued involvement in the story would earn Telltale the most money... And for good reason! People love her, after all.
TL;DR: Clementine still has a large amount of potential. Throwing her away now would make the past two seasons feel pointless and TWDG's heart and soul would be absent. It wouldn't feel the same.