One idea could be to explore Plankton's backstory. Maybe he wasn't always so obsessed with the Krabby Patty formula. He could have had a normal life before something happened that made him turn to villainy. Another idea is a story where Plankton teams up with other characters from Bikini Bottom in an unlikely alliance. For example, he could work with Sandy to achieve a common goal that has nothing to do with the Krusty Krab.
One significant archaebacterial group from marine plankton is Thaumarchaeota. They play crucial roles in nitrogen cycling and are abundant in various marine environments.
First, think about the aspect of Plankton you want to focus on. Is it his personality, his relationship with other characters, or his goals? Then, come up with a basic plot. For example, if you want to focus on his relationship with Mr. Krabs, you could start with a scenario where they accidentally find themselves in a situation where they have to work together. Next, start writing the opening scene. Describe the setting, like the inside of the Chum Bucket or Bikini Bottom street. Just let your ideas flow and don't worry too much about grammar at first.
We don't have enough information to know exactly who Karen is. She could be an ordinary person, a student, a teacher, or just someone who likes to read stories.
Given only 'dateline karen read story', Karen's identity remains a mystery. She might be a staff member at Dateline who was reading a story for work purposes. Or she could be a person who was interviewed by Dateline and was reading a story during the interview, or she could be an ordinary person whose story - reading experience is being highlighted in relation to Dateline.
It's hard to say for sure. 'Karen' could be a blend of real incidents and creative imagination. Sometimes, such stories take real-life inspirations but are embellished for entertainment value.