There will be more difficulty levels leading up to release. I know this because I saw an option for difficulty in multiplayer, but there's no way to actually select it yet because they haven't been implemented.
My only hope is that they will actually be hard enough. Divinity OS2 added two (or was it three) difficulties EASIER than classic. I was pretty shocked by that considering how easy classic was, but I guess enough people are bad at strategy games that they had to satisfy the demand. Meanwhile, the hardest mode was only one above classic (tactician, or honor, same difficulty really). I had to mod the fuck out of OS2 with difficulty mods to actually make it challenging enough to not feel trivial. I sincerely hope they add an actually difficult, like one that may even feel unfair, to the game for those of us who really want to grind a challenge.