This obviously depends on what you mean by an "open-world". To only have one very closed area at a time and only let the player do certain things in a extremely predetermined order are often bad game design if you ask me, regardless of genre.
The difference between an adventuregame and a RPG is basicly that in the first genre you interact with characters and the gameworld in order to solve different variations of puzzles to proceed. In the latter you fight enemies and gain abilities that the player can use to build ones charater in order to choose how to fight your way through the game. An "open-world" has nothing to do with either of these genre definitions IMO.
If you think about it there are already several adventuregames that let the player have access to many different locations at once. Monkey Island 2 did this and more recently Stacking.
How to tell an engaging and detailed story while still letting the player choose how it evolves is a conundrum that games have battled with since the start. We have seen examples of both boringly linear games as well as incoherently open games.
The trick is to have both and the big downside is that this often costs way to much money. Although there are many examples of this (Outcast anyone?) Stacking shows a way that it can be done with a rather modest budget. In other words, the future for adventuregames are looking good.
