Update: At the BlizzCon Overwatch panel, Jeff Kaplan confirmed that Overwatch 2 will be released for the PC via Battle.net, Xbox One, PC, and Nintendo Switch.
Blizzard's newly announced Overwatch 2 is both a sequel to the original Overwatch and an extension of the first game's evolving PvP experience. Game director Jeff Kaplan knows that isn't the clearest explanation, and provided some more details on stage at BlizzCon 2019. Thankfully, the accompanying press release for Overwatch 2's announcement goes a bit further in laying out what players of the new game will get from the old, and vice versa.
First, the press release confirms that "all of the original game's heroes, maps, and modes—as well as existing Overwatch players' accomplishments and loot collections," will be carried forward into the sequel. That brings Overwatch 2's confirmed hero roster up to 32 counting Sojourn, whose addition was teased alongside the sequel's announcement.
As for the "shared multiplayer environment" Kaplan described between the two games, the release notes that Overwatch and Overwatch 2 players "will battle side-by-side" in PvP multiplayer, and adds that "current Overwatch players will be able to play Overwatch 2 heroes and maps."
Beyond that, the release doesn't offer any more specifics on how this cross-compatibility works. While it's confirmed that Overwatch 2 PvP content will be playable in Overwatch, there's no word on whether or not there will be any limitations on how players of the original will get to experience this new content.
The press release makes no mention of Overwatch 2's PvE being compatible with the original, and mentions that the sequel will feature "significant engine upgrades that support larger maps for co-op (PvE) play." At BlizzCon 2019's dedicated Overwatch panel, Kaplan stressed that progression mechanics from Overwatch 2's story modes won't carry over to competitive multiplayer. In other words, however the two games work together, Overwatch 2 players won't get an unfair advantage from story content over original Overwatch players.
Blizzard has yet to confirm a release window for Overwatch 2's launch. Overwatch is available for the PC, Xbox One, PlayStation 4, and (as of last month) the Nintendo Switch.