This section contains spoilers for Kingdome Hearts 3D: Dream Drop Distance. BIG SPOILERS. Spoilers that give me hope and make me sob.
I have already described that I believe everything Terra did was for his friends and family. Sure, sometimes he went the wrong way about it, or he trusted the wrong person too much. But this is what gives him his inner struggle. He knows that his choices are not always good. He understands that he has to keep moving forward and accept responsibility for his actions. His whole journey is based on atoning for a mistake and trying to overcome a weakness in his heart.
When Terra meets Riku on Destiny islands, he sees a little boy that wants to leave and explore with his friends. He sees a little boy that would do anything to protect them. He sees himself. He and Riku make a promise as Terra chooses him as his successor. I feel like he honestly believed that whatever he could do, Riku could do better. If something went wrong, if he couldn't fight anymore then at least someone, somewhere could. Terra came quickly to terms with the idea that he may never be able to atone for what he'd done, but he didn't want to leave the unversed or even his friends completely alone.
That's what makes Terra's choice of successor so hard and intriguing for me. Riku follows in his footsteps almost perfectly. The only thing that they ever wanted was the power to protect those they cherished most. Unlike Terra, however, Riku mastered his darkness. When Riku is thrown into Sora's dreams he gives up everything to save his best friend. His journey through Sora's dreams and the disregard for his own life and well being are ultimately what lead Riku to obtaining the mark of mastery. Something that Terra never achieved and never could with darkness in his heart according to others. Riku is named a keyblade master, darkness not excluded because it is a part of him. He has mastered it. His power is seen as a benevolent force used to protect not harm. Terra was not given such freedoms or considerations. I wonder if Terra's plight is also part of what helped Riku in becoming a keyblade master. I feel like it certainly helped Riku and given the chance, Terra would easily master it for the same reason: to protect those he loved.
This works two fold for me. One, if Riku can master the darkness and become a keyblade master so can Terra. Terra could achieve his dream! He could protect his friends. Two, light and darkness exist together so that nothing is forsaken and the world is in balance. On the other hand, I have a very bad feeling that Sora and the others will save Terra and he will die (because he's been bathed in darkness for at least a decade) or they will be unable to save him at all. In special scenes we see Terra with Ven saying Sora with a smile, leading me to believe that ultimately Sora will save everyone. However, it worries me that saving does not always have to mean that the person saved lives. I'm certain that Sora and Riku will do everything within their power to save Terra. I just wonder what that means ultimately.