A Walter Koenig cameo on Star Trek: The Next Generation would've been awesome

Star Trek: The Next Generation hosted many stars of its predecessor series, the now dubbed Star Trek: The Original Series. James Doohan, DeForest Kelley, Leonard Nimoy, and regular actor Mark Lenard all appeared on The Next Generation, while William Shatner would appear in the first film, Star Trek Generations. Another long-time original series actress, Majel Barrett appeared on the Next Generation in various roles.

Meaning that the only major names from the original series who didn't were Nichelle Nichols and George Takei. Walter Koenig, much like Shatner, appeared in the first Next Generation movies, Generations, but unlike Shatner, Koenig was close to guest-starring on The Next Generations

Had things gone differently, Koenig would have done just that, telling Star Trek.com;

"I met with the people at 'The Next Generation.' I met first with Brannon Braga. I don't remember if they invited me or if I suggested that we get together and talk about me doing a guest role. Then he wanted me to meet with the entire writing crew. We were in discussions about what this appearance could be because there were restrictions, like no time travel. Then the meeting was canceled, right in the middle of the meeting itself. They were getting to the end of their last season and they were also preparing for the finale."

The actor could have done both the show and film, but the issue seemed to center around how to work his character into the series. Doohan's Montgomery "Scotty" Scott appeared due to being stuck in a form of stasis and not aging for 80-odd years. Kelley's Leonard "Bones" McCoy appeared as a very old version of his iconic character. Shatner used time travel for James T. Kirk, while Lenard and Barrett played entirely different characters.

So having Koenig on the series without some sort of time travel, or age-advancement makeup seemed to be a hard ask.

Why there were restrictions is anyone's guess, as is what those restrictions actually were. They could've just done something similar to Kelley, where they would have aged up the then 57-year-old actor. It probably would've worked, as he was supposed to be the youngest member of the crew when he was first introduced, so he could be still quite young and spry when The Next Generation was going on, at least compared to Kelley's Bones McCoy.