I hope you've managed to save up some in-game currency, because Overwatch's third Anniversary event will be arriving this May 21st. No official end date has been announced just yet, but given that the previous events stuck around for three weeks, I think it's safe to say this Anniversary event will end around June 10th.
As far as content is concerned, the Anniversary event will bring with it a bunch of new cosmetics (including a D.Va skin that is so good it'll "break the Internet"), all of the previous seasonal brawls, as well as all of the previous event cosmetics. And yes, this does include the most recent Storm Rising event. So if you have any achievements you want to complete or skins you want to collect, this would be your last chances for the foreseeable future.
When it comes to the new additions, nothing has been announced just yet besides the supposedly super-duper-ultra awesome D.Va skin. The only thing I can really say with 100% certainty is that you'll see dance emotes for the three characters that are currently missing them: Wrecking Ball, Ashe and Baptiste. Out of the three, I'm definitely curious about what they'll do with Wrecking Ball as his model and personality are quite unique!
That said, Blizzard has previously announced that they are working on multiple features that have been highly requested by the community, including one that is incredibly useful yet often forgotten by the playerbase. What exactly these features might be, they obviously didn't say, but there is a decent chance we might see at least one of them arrive alongside the Anniversary event. After all, it is Overwatch's biggest event of the year, so might as well mark it with something truly spectacular!
Once more information becomes available, most likely when the event itself starts, I'll make sure to let you know. Until then, you can read a little bit more about the previous Anniversary event over at the official website.