Larry, Darryl, and Darryl appeared in the second episode of season 1, “Mrs. Newton’s Body Lies a Mould’rin in the Grave.”
I much preferred the season 1 cast. Kirk (who was in seasons 1 and 2) was great, and I liked Leslie. Stefanie and Michael were just too shallow for me.
Cousin Oliver
No. Just no.
Jason Priestly in Tru Calling. Before he showed up episodes were kind of same-y. Tru would meet someone who dies (or some other tragedy) and goes back in time to save them. She usually (always? I don’t recall) messed up in her 1st try.
Jason’s character (Jack Harper) actively tried to prevent Tru - he wasn’t necessarily evil - he was just preserving “fate”
Brian
Pootchie.
Hill Street Blues - Dennis Franz was cast to play a short term character named Sal Benedetto on the show. But he was so obviously talented that after Benedetto was killed, they brought Franz back as Norman Buntz, a new character who was basically Bennedetto reborn. (And then brought him back again as Andy Sipowicz in NYPD Blue.)
I saw what you did there. 
Maya on Space: 1999. Better acting than most of the rest of the cast, and something exotic and mystical and different, to (try to) boost the show from the doldrums.
Heather Locklear was a late addition to season 1 of Melrose Place and wound up becoming the star of the show.
It’s early days yet, but I think Trubel is adding something to Grimm (am I the only one who watches that show?) and Rosalee in the same show was a nice addition.
For Buffy, Faith was a good addition.
And on a related note, remember that Fred herself only came onboard at the end of Season 2.
Misha Collins, unless we have been watching different shows. I agree that the character of Castiel adds a lot to the show.
I think everyone sees what he did there, and he really should close the blinds.
Benjamin Linus on Lost
Marlo and Chris on The Wire
Gus Fring and Lydia Breaking Bad
Cameron Mitchell (Ben Browder) and Vala (Claudia Black) were great additions to Stargate SG-1.
Fraiser was supposed to only show up a few episodes in season 3 as Diane’s love interest. Shelly Long hated him and the writers thought she was a bitch so kept him on to annoy her.
Worf
Root on Person of Interest. Not sure about Shaw, she is just sharing Reese’s role, but Root has pulled the show into interesting directions. I’m not sure if that was the plan, or just happened organically.
She also taught us some interesting things. Like… that even alien cyborgs shave body hair and wear fashionable hairstyles.
Christine was much more interesting than that black lady she replaced (Billie?). Also, Roz added more to the show than either Selma or Flo.
I almost think of this as more of a shark-jumping moment, since it was kind of an implicit admission that the show was having trouble standing on its own two feet. The show did improve, but this was due to the “Dominion War” story arc, not because Worf added all that much.
Sorry, but dear God I hate Root. I really hope she gets killed this season, even tho I kind of doubt it.