The characters you choose can be anyone that you like as long as they’re not super popular and/or have their own GOOD solo movie (i.e. Batman, Superman, Wonder Woman, the Flash).
One idea I personally had was a Martian Manhunter origin movie that followed J’onn’s life on Mars until he lost his family and retreated to Earth. I think the main villain could be J’onn’s evil brother Ma’alefa’ak/Malefic alongside the White Martians. You could even have M’yrnn, J’onn’s father, be a mentor-like character to hopefully make the viewers care about him only to be killed at some point later in the movie by Malefic to make him a villain that strikes on a more personal level. In fact, it could even at least try to make us care about J’onn’s family to add an extra level of sadness to his character when they inevitably die.
My favorite idea I’ve had was for an Abin Sur movie that follows the titular character through some sort of adventure, preferably with Atrocitus as the main villain because I like Atrocitus and I feel like he doesn’t get enough attention. Eventually, Abin Sur would crash land and the movie would of course end where Green Lantern’s origin starts with Hal Jordan discovering Sur and him giving Hal his ring before dying. I like this idea because if done well, it could make you actually care about Abin Sur’s death and make him feel more like an actual person rather than just an expendable alien that has to die so that Hal can become Green Lantern.
I’ve got a ton more of these including an idea for a Deadshot movie and an Alfred Pennyworth movie based when he was in the military, so if you’re curious, just ask in the comments and I’ll try to get back to you as soon as possible.
One more thing, if you have any ideas as far as what a movie for your chosen character could be about, feel free to share. Be sure to include ideas for antagonists and other characters like mentors, love interests, rivals, family, and companions.
Nightwing
I’d like to see a Deadman movie. Or something with 3rd gen Starman.
I think a direct to TV Movie of Booster Gold would be neat. They (DC) could probably profit from Booster’s consumerism appeal for the commercials and scenes in the movie. It could be a time loop similar to groundhog day. Booster can’t stop the loop unless he becomes a hero not a spokesperson. Antagonist could be his twin sister Michelle Carter. In this version she (Michelle) is a successful superhero who came to the past before Booster.
Doom Patrol has some interesting characters that might do well down the road on the big screen.
Static Shock and Zatanna
I’d watch a straight up crime thriller, where the protagonist is chugging some weird soda, and it’s only at the climax that you find out he’s the Elongated Man.
I want to see a Warlord movie, I don’t care if it’s love action or animated. I would love for the lost kingdom from Aquaman be Skartaris. I really have to double check this but I think in issue 17 of the original series has a ancient Atlantian computer in the story. It could just be a Elseworlds story and not connected to the DCEU I would be just as happy.
Captain Atom
Dr.fate
Plastic Man
Green Arrow
Martian ManHunter
Nightwing
Wonder twins
Agree with Martian ManHunter. His stad alone movie would be a different view on the DCU. In that same movie bring in green lantern investigating what happen to the Martian race. Just a thought. 
The core 3 of the Legion Of Superheroes. It would be difficult without a big budget to have, not only a future, but multi species show, but I’d love to see Rokk, Garth, and Imra get some live action love more than they have
I would love a Jaime Reyes Blue beetle movie or animated showed.
Although i don’t think he is “less popular” Nightwing could make a great solo movie if they went with the feel of the 96-09 run. An Arsenal/ Red Arrow movie could be stellar if they wanted to do a dark story about his drug abuse. Constantine deserves another shot at a show I think. Jonah Hex definitely could get a better movie then the one with Josh Brolin. Deadman could be cool. The Question could be interesting.
Ambush Bug (The Movie) coming Christmas 2019
Zatanna but only if they kept her original costume with the magician’s garb and not the atrocious 1980s one.
I would love a Martian Manhunter movie but most movie goers wouldn’t understand his character. He’s one of those hero’s you would want to read his books to appreciate the movie.
I think Dr. Fate would be a successful movie if done correctly.