Pretty sure WW84 was set for a December 2019 release, then moved up to November when SW was delayed to December.
'Wonder Woman 2' Release Date Moved Up 6 Weeks
Then WW84 got pushed back to June next year due to being so close to Frozen, and it's a similar date to the first WW, as well as giving Patty plenty of time (she took a while with the first one)
But I was just thinking about this last night, since someone already mentioned it -
When did crazy Grace mention the Flashpoint plot to WW84?
When was the delay?
Are there any reshoots?
I think they could totally make the villain from the present day or the future, returns to 1984 and causes Diana to create a new reality, so the what already happened happened, but we get another reality where Arthur, Billy and Harley's stories stay the same, but you get new Bruce, Clarke and Barry......it might only require an easter egg or two or a scene or two in the movie to set the path for this, and they don't have to mention this again in the future....
Edit: and no DC doesn't need to do multi-verse, they could just recast and move on, have the WW84 play and move on, or ya do multi-verse because Marvel has deceived the audiences so many times in the past, eg Mandarin, Hulk in IW trailer etc...