I'm not interested in or even remotely excited by the prospect of reserving harder difficulty modes for DLC or later release. Difficulty settings are pretty standard stuff you expect to find in games, and I want to have an enjoyable experience the *first* time I go through the game. I don't want to have to go through the game several times before I can finally access a difficulty mode that finally offers me the challenge I've been longing for.
This is why I'm *sooo* happy that the newest version of Diablo 3 now offers difficulty settings right from the start, rather than the traditional mode for that franchise they originally used (which required you to beat the game first on an easier difficulty level before being allowed to access the next tier of difficulty). When Diablo 3 first released I was forced to go through the whole game on what felt like kiddy-mode. I never even came close to dying once. And by the time that was over and I was given access to the next difficulty mode, I was already bored of the game and repeating it wasn't helping.
Divinity : OS needs, in my opinion, to have all its difficulty settings in place for launch. You get one chance to make a first impression on players.. and if you leave them thinking that the game is boring because it's too easy, then you're probably not going to be able to convince many of them to come back if you add or adjust difficulty later on. Likewise (and for the same reasons), you need to offer a setting that is easier for players who are finding the game too difficult. That's why we need a range available from the very start.
Now, before someone argues "ah-ha! but first impressions counting is exactly why you need to delay difficulties until you can get them right!", I will just respond with "No". Delay the release of the game, not the release of the difficulty settings. Because delivering the game in pieces is not going to give a good first impression for the overall game. You can always make the difficulty settings better. Balance adjustments happen post-launch all the time. If the release of the game can't be delayed then you always have balance patching to fall back on. "Fall back on" meaning "last resort", though.. not a shortcut safety-net.