3. Data
Firstly, Brent Spiner has ruled out ever appearing as Data again because he feels that he has aged out of the role. He wanted the character to be killed off in Star Trek: Insurrection and finally got his way in Star Trek: Nemesis. But a backdoor was created to the resurrection of the character, by having his memories transferred to the android B4. Spiner returned as Arik Soong in Enterprise, in some of his best performances. But hear me out. Because the way to alleviate Spiner's concerns about being too old to play Data is to make Data appear older. So why would Data appear older? I'll tell you this love. Now the emotion chip installed on Data was lost with his body, but you can be sure the he and La Forge made many scans of it prior to installing it in Star Trek: Generations after all, it was previously broken and someone had to have fixed it. So they should be able to create a new chip. With that new chip, Data has the ability to love like a human and also feel sentimental and guilt. So in that sort of situation I think all it would take is for Data to enter into a long term relationship. His love begins to age, and after twenty years appears quite older than Data does he realises that this is affecting his relationship and be feels guilty. As seen in the final episode of The Next Generation, Data can change his hair. So why not his skin? Up until now he hasn't had a reason. Love makes people do crazy things, and one of them is to want to grow old with the person that you love. I find the concept of Data aging for that reason quite romantic. But then again I really liked Bicentennial Man. Mind you, you could also take that entire concept and equally apply it to Robert Picardo's The Doctor.