I believe You can not revisit levels You have completed. I have finished this game, sometimes it even locks doors behind You on level You are and cut You from backtracking it - like it Jail level if You go straight to lower floors of that map You won't be able to back till You defeat that level boss. Sometimes You can hack doors with tool to open them.
It would be totally cool if there was option to revisit completed levels - for example there could be security elevator that would allows for that. Additionally there could be option to repopulate cleared levels with new enemies, it could be totally optional. Think about different random encounters, if you clean sewers from slimes it could be occupied by rats. If You clean laboratory it could be next time full of security soldiers and raptors.
Another cool possibility is to have scenarios when you can find items that will unlock certain parts in earlier levels.
For example if You success in killing main game boss You could earn pass to enter Department of Lost Content.
Or if You find secret item in laboratory You could unlock in sewers secret enter to caves under tower. I am sure devs would have lot of cool ideas for places to visit.
