TV shows and movie series (like Star Trek) add characters and locations as stories and plots evolve. In some rare cases they also take characters away (they killed Brian on Family Guy). I was so sorry to hear it, because Brian was my favourite character on the show. Whatever the reason(s), I have to presume they were good ones, but the whole notion of augmentation is a really complicated and dangerous one. That's because there are so many pitfalls and ways to diminish a show's audience.
Not only can you get into dangerous territory by bringing in unanticipated celebrity actors to play supporting roles, you can also derail historical consistency. I still can't forgive Paramount for inventing the concept of a queen for the Borg (Star Trek: First Contact, 1996). How can a cybernetic collective of drones have a leader? They had been brilliant for years without one. It makes no sense to me, but I can see why they did it. They fell into the romantic trap of companionship to which all anthropomorphised, non-sentient characters are vulnerable. Look at Wall-E and EVE. What chances do those two little robots have for a longlasting relationship? Animated characters like Fred and Wilma Flintstone would have a better chance. Superhero comic book characters like Benjamin Grimm (AKA The Thing - Fantastic Four) need a love interest too. Although he was a victim of cosmic ray bombardment and turned into a grotesque superstrong lumpy, rocky freak, he still remains faithful to his blind girlfriend (Alicia Masters) - girlfriend? How romantic can that be?
On a more traditional dramatic stage, Captain Kirk has a relationship with a molecular biologist (Dr. Carol Marcus) with whom he has a son, as revealed in Star Trek II: Wrath of Khan (1982). We get to meet with her again with an all new cast in Star Trek Into Darkness (2013). At least Carol is human and not an alien like Mr. Spock (although he IS half human). Would Superman and Lois Lane have generated a brilliant hybrid offspring with a compromised mental faculty? What if a Metron got romantically involved with a Gorn? I dread to think what sort of childhood that poor kid would have. The marriage would probably break up and their offspring would get adopted by two gay male wookies and turn out to be an intergalactic tribble dealer with a taste for Romulan Ale and green Orion dancing slave girls.
Come to think of it, in terms of Vulcans, would they have been OK with Spock's father marrying an Earth woman? Which one is the alien? Ambassador Sarek or Amanda Grayson? How could their relationship possibly be condoned on planet Vulcan? What about the seven year blood fever of Pon Farr (as depicted in the original series episode Amok Time). How do we rationalize that concept for Sarek and Amanda or was he having more Pon Farr than the other Vulcans? Can you imagine if he cheated on Spock's mother and hooked up with one of those girls from Orion? Then Mr. Spock would have a green half sister with pointed ears. If I was an executive at Paramount I would give her a spinoff series of her own.