Quickly browsing through the forum pages revealed that there's no topic about how exploration will be handled. in this regard only baldur's gate was mentioned and i think that's because of its non-linearity. we know that they're thinking about camping:
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I've talked with Tim about this for a while and here's the thing: camping out in the wilderness and setting watches and getting ambushed by jackasses has a great classic A/D&D feel to it, but it got pretty silly in games like IWD2. I'd like to build in reasonable mechanics that make you rest in the wilderness, but I don't want it to result in the sort of degenerate "rest after every fight" stuff we've faced in the past.
i can think of 4 different system
1) a traditional 2d map where several locations are selectable (e.g. iwd, bg, nwn, etc.)
2) an interactive 2d map with free roaming as a partycursor; perhaps even with a fog of war (e.g. arcanum, fallout)
3) an explorable 3d overland map with skill usage and the party leader as representation (nwn2 soz)
4) as onyx is capable of world streaming even an open world would be possible
personally i would like to see the return of the third option. there was a lot of work put into this system and it felt kinda nice to choose which party member would fit the needed skills to overcome different situations.
last and most important question: WILL THERE BE MOUNTABLE HORSES?