>>40335
You can look up stuff about Earth-Two, which is the original DCU from the '30s and '40s. Batman, the original Batman, died in the early '80s. Catwoman died shortly before, and Robin and Huntress died a few years later. The entire Batman of Earth-Two story, with all its characters, gets pretty well wrapped up. Wonder Woman and Superman died later. Superman's whole cast of characters also got somewhat well wrapped up. Wonder Woman got into some more complicated stuff, where she had a daughter who continued living in the New Earth created in the '80s, but uh... Well Superman and Batman died and you can just consider their stories fully over, with no major hanging threads. A lot of the C-list guys have died and stayed dead over the years, too. But a lot of the B-listers, like Flash, Green Lantern, and Hawkman, are still alive. When was the last time the original Doctor Fate was alive? I know Kent Nelson died in the '80s or '90s and was a spirit for a while who only sometimes interacted with later Doctor Fates, but did he end up coming back to life ever? I just expect that he did.
The original Supergirl died so hard she got erased from history and nobody knew she ever existed. She later appeared as a ghost one time (maybe twice, I forget), teaching a lesson that it doesn't matter if people know you did something good, because the important thing is that you did it. But anyway you can read all of the original Supergirl's stories and get an ending. She never came back. After that ghost story they did say she could finally be reincarnated, and a new Supergirl with her name appeared, but for all intents and purposes, she's a brand new person with a brand new history. It's not like she or anyone else remembers the old Supergirl.
You can read every Superman story in order, and while it technically continues after The Crisis on Infinite Earths, they did do an ending at that point. It was non-canon, which was acknowledged in the foreword. "This is an imaginary story. Aren't they all?" Anyway, Whatever Happened to the Man of Tomorrow is a great ending.
The idea of The Dark Knight Returns is basically "what if we took Batman, pretended he was as old as he would be if he aged in real time, and then skipped the years between now and then and showed what he's up to?" So you can read all the Batman comics to that point and then skip to The Dark Knight Returns. Immediately after, the same writer, Frank Miller, did Batman: Year One, which was canon to the DCU but also intended to be the origin of Dark Knight Returns Batman. He eventually did other sequels, but also All-Star Batman & Robin The Boy Wonder, which takes place between Year One and Dark Knight Returns. So if you want to only read the things that are explicitly canon to this Batman, you can read Dark Knight Returns, Year One, Dark Knight Strikes Again, All-Star Batman, and then the other sequels Miller did later. Now, I won't vouch for the quality of those sequels, but they're there.
It's also notable that Dark Knight Returns mentioned Jason Todd being killed in what would be our present. This then happened a couple years later in the mainstream comics, so there was a good while there where you could consider everything to be leading up to Dark Knight Returns. Eventually, though, modern Batman stories started using elements from Dark Knight Returns, like Carrie Kelly, and the Mutants, but they weren't the same. It was like an alternate timeline. Batman isn't old yet though because of time shenanigans and things like Lazarus Pits. So there's a point where the DCU diverges significantly from Dark Knight Returns, but there is a point before then where you can consider DKR to be the future of main Batman, if you want. And if you don't, All-Star Batman fills in that gap.
Jonah Hex, being a character who exists in the past, has stories that take place all across his lifetime, and even after his lifetime, dealing with what happened to his corpse after he died. And yes there are some incidents of time travel and other times he gets involved with the DCU, but mostly he does cowboy stuff, and the rare times he does other stuff are cool because they're rare. Also it's cool that he's pretty much wholly unaffected by changes in history. Everything he does is canon. He's not the type of guy to wax on about his history much, but sometimes that's the reference. Like when a time traveller shows up, he probably won't be surprised, since it isn't the first time. Anyway you can read every Hex story, and pretty much follow his whole life. But since he's a historical character, they weren't all released in chronological order.
"The Life Story of The Flash" is a graphic novel that is what it says on the tin. After Barry Allen died, and his wife Iris was living in the 30th century, where she could talk about his private life without fear of supervillains (even though she should be fucking careful given that his arch-nemesis who fucking killed her a few years earlier was a time traveller from the 25th century). She was a journalist, so she wrote her husband's biography. The book exists in-universe, but DC also published it in real life, and you can read it, and it's good. Now yes, Barry came back to life like 15 years later, but this story works on its own either way.
The Ultimate Marvel universe (I know you said DC, but close enough) pretty much has a beginning, middle, and end. The end wasn't very good, but it's there.
"Marvels" is a graphic novel which basically looks back at the history of the Marvel universe up to that point (the '90s) through the eyes of a regular guy, telling it like one story. It's excellent.
I've heard X-Men: Grand Design does something a bit similar to Marvels but later. I never read that though so I don't know how good it is. Also Marvels is partially awesome because of Alex Ross's art, and X-Men: Grand Design doesn't have that.