>angry New 52 Supergirl
I never saw her as particularly angry, but I did think her series was really good. The bit where she actually couldn't speak any language other than Kryptonese until quite a few issues in was a pretty neat idea, and they did a good job making you feel for her situation, I guess because her narration did a good job at showing how alone she felt. That might be hard to adapt to a movie, though. The fact that she spoke with an accent even after learning English (and I think they even reference it well into the Rebirth era), but I imagine a movie would butcher that and just make her into Starfire from the Teen Titans cartoon.
>Animal Man Swamp Thing crossover
This was awesome and I'd love to see a movie about it. But as you pointed out, you'd need to establish who Animal Man and Swamp Thing are, since casuals aren't very aware of their stories these days, despite Swamp Thing getting two movies, a cartoon, and a live action tv show back in the day. I actually think the second Swamp Thing movie is pretty great, but it isn't exactly the tone that modern Swamp Thing is known for.
Also, don't forget Frankenstein. Frankenstein was in that crossover, too. I really liked his series. Gotta read more of his pre-Flashpoint stuff. Plus, Agent Frankenstein and the Creature Commandos would be an awesome movie regardless. I wish we got more stuff that explored those weird little bits of the DCU that casuals don't know about. Wonder Woman does a movie about a world war and it isn't world war II? They could have done The Creature Commandos, GI Robot, Dinosaur Island, the original Suicide Squad, the goddamn Justice Society. Instead we get World War I, and they don't even have the decency to mention Enemy Ace.
I really loved Forever Evil, and thought it was an even better Crime Syndicate story than Earth-2. One of my favorite universal crossovers ever. However, I don't know if they could condense such an absurdly long crossover with so many tie ins which feature all the members of the Crime Syndicate into 80 or 90 minutes without losing a lot of what made it great. Also, I think part of why I liked it was all the autistic references to deep DCU lore, including previous iterations of the Crime Syndicate, and that probably wouldn't transfer well to other media.
I actually appreciated the Death of Superman movie, since it's the best adaptation of that story so far. Things like replacing Mongul with Darkseid make total sense, and most of the cuts for time make sense too. It's too bad that a lot of the best stuff from the comics is the "World Without a Superman" part, which was not adapted to the movie, but again, I can understand why.