Overwatch 2 just made its official debut at BlizzCon 2019, making a grand appearance with yet another excellent animated short. Based on what can be told so far, it's essentially a PvE expansion to the base game. The idea behind it is that Overwatch 2 will have a story based campaign of sorts that advances the lore of the Overwatch universe. To avoid fragmenting the playerbase, Overwatch 2 will also contain the regular Overwatch's PvP mode. Any game modes, maps, and Heroes added to the game in Overwatch 2 will also be available to Overwatch players. Skins, sprays, and other cosmetic items from the first Overwatch will be carried over to the sequel. Servers and other services will be shared so that regular Overwatch players can play with Overwatch 2 players.
Though Blizzard probably won't like it, you can think of Overwatch 2 as a massive DLC pack. You're not leaving anything behind by buying it, but you're also not getting an entirely new game per se. At worst, PvP-only players will miss out on story development and skins. In fact, pre-release footage from BlizzCon gives the impression that the game is largely based on the seasonal PvE events from the first Overwatch.
As far as can be told, the largest change that Overwatch 2 brings to the table are Hero abilities that you can upgrade. Beyond that, the basic mission structure will be very familiar. You choose from a preset group of Heroes, kill Null Sector Omnics or Talon forces from point A to B, and then make a dramatic escape. It is almost a guarantee that the entire game will be co-op only. Single player will likely not be an option, so you might want to make some friends now before you're forced to play with randoms.
On paper, there's not really much of an incentive to pick up Overwatch 2. The PvE events from the first game were fun enough, but they're quite repetitive. You could make a rather safe assumption that most people played the events for the rewards. People certainly aren't playing the events for the story given how quickly most lobbies tend to skip the introductory cutscene after the first day or two. Most games that are based around repetitive gameplay where you fight dumb AI bots with random people tend to have very short lifespans, or are sold as looter shooters. This is, of course, going to be the largest obstacle to Overwatch 2's success.
On the other hand, it would be incredibly strange for Blizzard to throw Overwatch 2 to the wolves without some kind of incentive to play it. It's no secret what people's opinions are regarding the PvE events. True, Blizzard is money hungry, but their actions so far don't quite reek of desperation. That there's no release date furthers the idea that Blizzard is holding some kind of ace in the hole. Perhaps they're working on some new gameplay mechanics for enemies or significant engine upgrades that require the power of next gen consoles for optimal performance (though this is more of a pipe dream than anything else).
After all, it's not like there haven't been similar games in the past, at least conceptually. Left 4 Dead for example kept things fresh by having different zombie spawns every time you load a mission. The general pacing of the missions were consistent enough across playthroughs that you could more or less commit the maps to memory, but whether or not certain special zombies spawned and where they spawned was entirely up to the game. The same kind of mechanic can be applied to Overwatch 2. Major enemy spawns would remain the same across playthroughs, but the variety and intensity of normal enemy spawns could vary greatly. Forced cooperation is already a central facet of Overwatch gameplay, so it's not like Blizzard has to reinvent the wheel or anything.
Regardless, it's hard to imagine that Overwatch 2 will drastically shake things up. For starters, the game can't conceivably compete with dedicated narrative based games. PvP balancing likely isn't going to be more responsive overnight. From a narrative standpoint, it seems doubtful that any playable Heroes from PvP will be killed off. Amusing as it would be if that is how Blizzard decides to remove some of the more problematic Heroes from the roster, the precedent it would set would be controversial at best. There's little doubt that Overwatch 2 will make money of course. The real test will be whether or not the game will have any staying power when the PvP is the main draw of the franchise.