Map icon is based on the type of encounter, not difficulty. Although I do remember some sort of skull so maybe there is a warning indicator for discovered, tough areas.
Either way, I think the map is pretty well designed so that the nearby quests are level appropriate. There are definitely a few tough spots in Velen that are intended for you to revisit later, but most of it is quite easy. The real problem there is that there is a low-level quest to
that requires you to walk through the most difficult part of Velen to complete.