Rivers dry up, mountains erode, and shows come to an end or are cancelled. Such is life.
Often, we don't want these shows to end. And in the streaming era where new subscribers are driven by new shows (and old shows just run up royalty costs for the streaming services), corpos seek to maximize profits and reduce costs. This means good stuff gets cancelled, too.
So what do you do when a show ends on a cliffhanger, or has an obviously rushed ending, but you want more? One option is to look at the other stuff that the show creators have made.
Take "Rise of the Teenage Mutant Ninja Turtles," for example. It could've easily gone for another season or two, with the Kraang threat extended into a season instead of a movie (or even have a follow-on to the movie). There were plenty of loose threads. Well, you could look up fanfic and fanart (which may be good if you look for the top 10% of stuff, but suffers even more from early cancellation as a labor of love), attempt a petition to revive the show (ha!), or die mad about it.
Another option is to acknowledge that all things end, even if they end too soon. But you still. Want. More.
The first step here is to look up the other stuff that the creators have made. A quick check on Wikipedia shows that ROTTMNT was developed by Andy Suriano and Ant Ward. Neither of them are fancy enough to have their own Wikipedia page, but they do have IMDB and instagram pages.
Sometimes, you get lucky enough that creators will share "behind-the-scenes" stuff on social media, too.
Otherwise, it would behoove you to look at the other things that the creators have made to see what else you might want to watch. In this case, Andy Suriano has a long list of things that he's worked on since 2001, and although Ant Ward's list is shorter, he has some more recent ongoing projects as well.
You could even start looking into storyboard artists, voice actors, and other folks who worked on the show to see what they might be involved in.
Hokay, maybe source one is a bit obvious.
The next thing you want to look at is the stories that inspired the creators of the thing you liked.
If the creators are big enough, they may have already discussed as much in various panels or interviews.
You could also try asking them via social media. As a millenial, this is still one of the things that's crazy to me in how easy it is to do. Go back 20 years and I wouldn't even have a clue how to find out who worked on a show if they weren't named in the credits, nevermind how to contact them!
Tropes are a bit more specific than genre. It's a good idea to have a little bit of knowledge of the tropes your favorite show uses, and which ones you like, if you want to find other shows like it!
Sadly, some of my favorite tropes (redemption arcs, noble demon, etc.) tend to be spoilery, so that can make it tricky to find similar works. But TvTropes is pretty good about spoilering the text of things that need spoilering.