Sandman technically already existed in the MCU, though it was an illusion created by Mysterio, denominated the Earth Elemental.
Doctor Octopus could have a much different origin. I would find it interesting if they went with a Carolyn Trainer iteration, where maybe she becomes a villain of Spider-Man, perhaps by using the same tech as Otto Octavius, who died, maybe, in some form of incident where she can blame Spider-Man, like London, or something.
Norman Osborn would be a guy who has injected himself, perhaps with some form of "supersoldier serum" or some form of thingie that makes him slightly mutated, but that makes him enraged and a public menace, killing someone in the public, and then he adopts the mantle of the Green Goblin, and Spider-Man has to fight the insane man.
Electro would be a regular worker in an electric company, and then gets struck by lightning and gains an exceptional electrical conductivity, allowing him to conduct and control electrcity through him. With his newly acquired powers he would have the motivations to turn into a life of crime.
Lizard, well, would be a gifted american scientist who ends up experimenting with a highly mutable serum that has lizard DNA, and then he transforms into a monster, and then Spider-Man has to fight him.
I personally do not think the origins of these characters would need to be too different from how they do in the comics and in other movies. Its pretty straightforward and predictable anyway.
As for my suggested casting:
Ana de Armas as Carolyn Trainer/Dr. Octopus
Dominic West as Norman Osborn/Green Goblin
Jeremy Piven as Max Dillon/Electro
Crispin Glover as Curt Connors/Lizard
