If I were to fix BvS, I’d do it the same way I’d fix MoS: I’d drastically cut the budget, thereby forcing some script changes. Snyder needed a good producer to rein him in and tell him that certain things weren’t worth the money they’d cost.
The Knightmare sequence is an enjoyable enough apocalyptic short film, but it’s not necessary for THIS story, nor is the Flash appearance afterward.
The ultraviolent car chase might have been less controversial if Snyder couldn’t have spent so much money depicting so much mayhem.
The Flash, Aquaman, and Cyborg cameos were absolutely unnecessary. (If you’re going to film clips like that, use them in a viral ad campaign. Don’t stick them in the movie.)
Doomsday had no business being in this movie, so I would tell Zack, “We can’t afford it right now. Save him and the death of Superman for the sequel.”
Again, though, the Ultimate Edition does so much to expand the rest of the movie that these issues weigh down the final product a bit less.