Since both the Witcher Games have a somewhat open world (but small), I think we shouldn't worry that much about it. ›››
Well, that's arguable.
I know many are going to disagree with me, but I'm not exactly enthusiast about how CDPR handled their "open worlds" so far. There's a LOT of room for improvements in their formula.
What I would like to see the most: an open world designed more closely to what Gothic 1-2/Risen did than to Skyrim/Mass Effect.
No invisible walls, many possibilities of interaction with environment and NPCs, less lifeless anonymous NPCs wandering around for the sake of it.
Really, Floatsom and Vergen were both two of the prettiest towns I ever seen in a RPG but they also felt more like "backgrounds" than part of the experience.
They were pretty to look at, they were colorful and nice, sure, but there wasn't really much you could do in those places.
CDPR should use Khorinos or Harbor Town as models of how to build a "quest hub". Places that despise their limited size felt lively and populated by unique characters, which offered unique interactions.
Also, beside NPC interaction, more climbing/crouching/sneaking/platforming, more vertical building less "walk on this flat landscape and click on the hot spot where the mouse pointer changes its shape".
Make the environment part of the gameplay, not just a passive backdrop that looks nice and little else.
Once again, like Gothic or Risen did before. Or Dark Souls, if you want a more recent example.