Wouldn't need to be a DLC; a patch would suffice.
I'd hazard a guess to say that since the engines are the same, and the game supports day and night modes, then day/night cycling should be "flick of the switch" (figuratively speaking).
Or as part of the island/content unlocking process.
Can see both sides of this; Understand why plunging a starter island into darkness, could be a problem if you're still figuring out the basics, yet would like to have the game turn into a sandbox, once you've completed the missions.. So I'd say I get it when starting the game but would like to see it unlock later.