If the previous Divinity game are any indication, D:OS will likely be balanced for a final level of 20, rather than setting an artificial limit.
Thanks. Now that you mention it, I think Divinity 2 had a level cap but it was practically impossible to reach it or something (because of the vastly decreasing XP rewards when you were over enemy levels).
I'm also hoping for balance. Higher difficulties should be such that you have play smart and turn pretty much every rock to find all the good stuff unless you want to have an extremely difficult time. On easier difficulties you necessarily don't need to reach level 20 to beat the game.