The world opens up once you finish the prologue, apart from that Skellige is locked off behind a short side quest. Unfortunately you'll still be gated off by every area having a level range, though tackling enemies up to 4-5 higher than yours is feasible.
@Zoraptor While it's true that main story quests give you a ton of xp (and I think they're not subject to diminishing xp if you do them above level?), I did them last in each given area and was still consistently over-leveled, to the point that -and despite- most quests gave me between 5 and 125~ xp.