Girl Genius starts with a few. The poet is telling the story, so he will definitely survive everything that's going on.
During the comic's run we get some short stories set in the future, which give us more. For example, from Ivo Sharktooth Private Jaeger we learn that Agatha and Van will survive, Agatha will free Mechanicsburg and rule there, and Agatha will ally herself with the Empire.
In the Nailbat story in AntiBunny It's established early on that the protagonist will die in one year. As the other AntiBunny story Gritty City Stories takes place two years later, Nailbat's death is already mentioned there as a matter of fact.
The Last Days of FOXHOUND: If you've played the game, you know how the main characters end up. At the beginning when it's all flanderizing the characters for humor, this doesn't register. At the end after a long bout of Cerebus Syndrome, it's pretty damn bleak. The panel with Sniper Wolf and Bertholt is exceptionally heartbreaking.
Long Exposure has in it's very synopsis the fact that Jonas and Mitch will eventually fall in love with each other. The author herself releases sketches and quick comics about their life as a couple, including a drawing that shows them still together by their early 30s.
The book "The Sharp End of the Stick" of Schlock Mercenary starts with several characters dressed in loincloths and wielding sharp sticks, rather than their usual military uniforms and plasma weapons, not to mention that Kevyn and Elf have become a couple. The rest of the story switches back and forth between telling the story in chronological order from that point and showing how the characters got there.
Chess Piece takes place during The Roaring '20s — 1927 currently, to be exact. Although times are good, the Great Depression is just around the corner.
Homestuck, all the time. Not only does the story run on Anachronic Order, but time travel and having visions of the future are regular occurrences, and twelve of the sixteen major characters with dialogue already know everything that's going to happen for a large portion of the story and regularly tell the four protagonists about it.
The current "Tower of Babel" arc of S.S.D.D is essentially the backstory of one of the characters, and previous arcs make it clear that Tessa's squad destroys Arthur, but during the battle Julian is killed and Tessa is captured. Then she escapes with help from Tin-head, and sometime later wins Sticks from Julie Waterman in a card game.
Spacetrawler: Nogg tells Mr. Zorilla that his daughter, Martina, has died. The rest of the comic is Nogg telling "the long and very detailed version" of how this came to pass.
Kick The Football, Chuck uses Charlie Brown attempting to kick Lucy's ball as a metaphor for his fight with cancer after chemotherapy. We all know he never kicks it.
Played with a little bit earlier before that; Sarda believes that the outcome of his battle with the Light Warriors is a foregone conclusion, and that he literally cannot lose to them, since his present-day self grows up in a world that isn't terrorized by the "heroes". He does actually lose the fight... sort of... but the Light Warriors disband after the battle anyway.
The "Sam" arc of General Protection Fault goes into Ki's past with Sam, her former fiancee, who had been alluded to in the past. While it is implied that they had a bad breakup, the arc reveals that he tried to rape her.
Much Erfworld's "Inner Peace (Through Superior Firepower)". The story is Wanda's loss of Goodminton, and her journey to be a caster in Faq. Anyone who's read the main story already knows that Faq falls through Wanda's actions.
In-story, anything foretold by a Predictamancer. Prophecy Twist is possible, but less common than you'd think, and is a source of endless grief for the main characters.
There is a multi-chapter flashback in Evil Plan which tells the origin story of Kinesis from Will's perspective. The entire time you get to know how much of a bright and happy spirit Will was, knowing the flashback has to end with his death by Stanley's hand.
One of the main draws of Sire. The Binding is a mystical force which forces the lineage children to follow the destiny of their sire/dam. Dramatic Irony itself is the antagonist of the series and each character just has to work their hardest to avoid their foregone conclusions.
We all know Gogeta will make an appearance. We just don't know when.
In-universe as a sort of handwave, Raichi from Universe 3 is competing to kill Saiyans, but none had joined the tournament by the time he did, since his was an earlier universe. This discrepancy is addressed by the Vargas who offered him a spot, mentioning that the Saiyans' warlike ways and powers mean they're sure to join from some universe.