William Riker was one of the coolest characters in all of Star Trek: The Next Generation. He was suave, smooth, and witty, making him one of the most fun characters to follow around the ship every week. Yet, he wasn't just there for his charm and smirk, as Riker has some of the best stories in the series' history. Why shouldn't he, Jonathan Frakes was a brilliant actor in the series, and he was able to go a lot of places emotionally while on screen.
This decides to not further explore one of Riker's best storylines even more maddening. In season six's third-to-last episode of the season, "Second Chances", we find out that Will Riker has a clone, or that Will may be a clone. Eight years before the episode, Riker was involved in a pretty jarring transporter incident. Everyone thought the incident came and went, as Will Riker was fine after all. Yet, in the episode "Second Chances" we find that this specific incident had an unexpected side effect.
See, when the transporter malfunction happened, it ended up creating a duplicate of Riker that no one knew about. Eight years later, the crew of the Enterprise would discover the unknown duplicate. The duplicate would reveal that up until the moment they were split, they had the same memories and experiences, making it unknown who the "real" Riker was, or if there even was a "real" Riker.
Eventually, the newly discovered Riker would go by Thomas to simplify things, but that wouldn't help the two men get along. Will wanted nothing to do with his counterpart and eventually, Thomas, still a member of Starfleet, would transfer off of the Enterprise to forge his path. He'd appear just one more time, now as a member of the Maquis, and never again.
The lack of a follow-up or even a deeper seasonal arc between Thomas and Will was a missed opportunity. Not only could you further look into the relationship between the two men as they figure out how to navigate space with one another, but you could've gone far deeper into his turn to the Maquis than just a random one-off episode on Deep Space Nine.
It was a missed opportunity to shuffle Thomas off of the Enterprise at the end of his premiere episode, and it would've made more impact had he stayed on the ship throughout the seventh and final season.