The colorful characters are what made The Big Bang Theory such a rousing success. The hot-and-cold bond Sheldon had with Leonard, Howard, Raj, and beyond was sitcom gold. Due to Sheldon's social awkwardness, the group had to teach him what was socially acceptable and what wasn't. There was nothing like seeing the connection between Sheldon and Penny, or even Howard and Raj, but there were other bonds fans wish the show had spent more time on.
Along with the core cast, there were supporting characters who also made a lasting impression. From Barry Kripke's intensity at work to Denise's familiarity with comics, it would have been exciting to see these characters interact more throughout the show's 12 seasons.
Denise & The Gang
Denise came on the show in season 11 after Stuart's comic book store started seeing success. He hired her as the store's general manager and she made quite an impact. She wasn't just a pretty face; Denise loved the comic book world and was very familiar with it.
When Stuart and Denise started seeing each other, she hung out with some of the characters on rare occasions, but fans wanted more. Watching Denise teach Bernadette how to play video games or teaching Amy about comic books was genius television. It would have been fun to see Denise at one of Penny, Bernadette, and Amy's girls' nights!
Professor Proton & Penny
Professor Proton started popping up in season 6. He developed a unique relationship with Leonard and Sheldon before he passed away. But even in the afterlife, Professor Proton continued to visit Sheldon in his dreams to guide him in his time of doubt.
It would have been hilarious if he and Penny had more scenes together. He had a small crush on Penny and enjoyed talking to her. How fun would it be to watch him give her a lesson in science, just as he would to a child? Maybe Penny would have finally understood what Leonard did for work!
Zack & The Guys
Penny saw Zack Johnson off and on in between dating Leonard. Zack was the complete opposite of Leonard in every way imaginable. As tall and handsome as Zack was, he was ditzier than Penny. Nevertheless, he claimed to be interested in science and loved hearing what the guys were working on. He asked genuine questions and clearly wanted to be friends with them. Seeing a guy like Zack try to work his way into Sheldon and Leonard's group of friends would have been fun to watch.
Janine Davis & Howard
Janine Davis popped up sporadically throughout The Big Bang Theory starting in the sixth season. She was the human resource representative for Caltech and she had one too many inappropriate run-ins with Sheldon, Leonard, Howard, and Raj.
And as hilarious as Sheldon's interactions with Ms. Davis were, watching her and Howard's private conversations would have been legendary. Before Howard married Bernadette, he was incredibly forward with everyone he encountered. He had to have had a rap sheet with HR.
Emily & The Girls
Raj dated two Emilys during the show, but his relationship with Emily Sweeney was by far his most serious. As different as they were, Emily truly loved Raj and called him her best friend.
They dated from season 7 through season 10 and oddly enough, fans never saw Emily mingle with Penny, Amy, and Bernadette on a more personal level. She was never a part of their wine nights where they vented to each other about their woes. It would have been nice to have gotten to know Emily more for the kind of person she was rather than just as Raj's girlfriend.
Kripke & Sheldon
Barry Kripke, a fellow Caltech colleague, was a frenemy to the group, but particularly to Sheldon. It was instinctual for Sheldon to loathe Kripke because they were each other's competition but they slowly became friends. Kripke was invited to a few of the group's events but fans wanted more. His sarcastic attitude and confidence made him a fun character on the show, yet he only popped up here and there.
Mrs. Cooper & Sheldon
Mrs. Cooper was a welcome addition to the show. A woman with old-fashioned values, Mary Cooper could be incredibly tone-deaf but she adored her "Shelly." With her living in Texas, she wasn't always around but her moments on-screen were memorable.
Sheldon was well-behaved with his mom around and she got along with everyone. She was honest but also very loving and nurturing. Seeing how happy and calm Sheldon was when his mother was around was special to see.
Stuart & Raj
When Howard became too busy with his wife and kids, Raj began leaning on Stuart. As much as Raj loved the Rostenkowski-Wolowitz family, he had more in common with Stuart as time went on. They were both looking for a place to live and were single for a long time. Seeing these two compare their "woe is me" stories would have been great comedy television. It would have also made them feel less lonely.
Beverly Hofstadter & Penny
Leonard's mother, Dr. Beverly Hofstader, was one tough woman. Like Sheldon, she didn't always pick up on sarcasm or different kinds of humor. She was far too bright and didn't enjoy the social part of life. Although she and Penny grew to love each other by the 12th season, these two didn't always get along. They had nothing in common and Beverly used to judge Penny for her career goals. Watching Penny and Beverly learn from each other would have been a blast.
Leslie Winkle & The Group
In many ways, Leslie Winkle was the female version of Leonard. They were both experimental physicists, were around the same height, and had a similar sense of humor.
However, unlike Leonard, Leslie couldn't stand Sheldon. She found him arrogant and gave him the nickname of "dumbass" when he didn't understand sarcasm. Leslie used to pop up more in the earlier seasons but diasppeared as the series went on. Her banter with Sheldon was some of the best parts of the show and fans wanted more.