I feel it's a weird one, on one hand, a hard reboot will disappoint those on the Cavill and DCEU train - I know I was happy for him to return, simply because he looks the part, he's a nice guy and I liked MOS.
However, a reset, recast could blow us all out the water.
For my money, I would retcon the the DCEU into the DCU.
The Flash movie resets it, have a post credit sequence where we see the pov of someone waking up - they are in a familiar room, it's Barry Allen's room, the flash suit is on the floor, the tv is on showing the news of a classic superman rescue. He gets ups and notices he 'feels' different as he puts it away - he moves different, he doesn't recognise his hands - he hears an inside voice, like his inner monologue - that even sounds different to him... he looks into the reflection of the TV as Superman addresses the crowd and says something hopeful and instantly Barry doesn't recognize his silhouette on the glass - he flashes to a mirror and stares in disbelief - he's a different Barry Allen. Recast done.
As a side note, earlier in the movie we see a scene in the speedforce with a reshot scene where we see multiple Barry Allens and this one is also out there - so this is the face/body he resumes in once the universe are merged.
Then, we have a booster gold movie and he breaks the fourth fall of sorts and we learn that the timelines were changed, thanks to the events of flash and that's why he has to obey strict time travel rules and that everything has merged into one universe - the dceu, the Burton verse, the arrow verse etc Just ignore the Pattison verse for now.
Then movie forward with the DCU fresh and pump out good movies. People will enjoy good movies, period.