how much do you reckon classes matter then?

Just as much as they do in PoE1. They only determine where you start in the skill tree and what ascendancies you can choose from, but you can still get to any point in the skill tree regardless of starting location, and the ascendancies seem to be designed to be very generic such that the Warrior ascendancies are the only ones that are specifically for melee. All the other ascendancies are super general and they will be playable with any given archetype (well, maybe a melee deadeye is gonna be hard to justify but I'm sure there's melee skills that launch projectiles like Molten Strike or Lightning Strike in POE1). The Sorceress ascendancies also seem fairly generic and will probably work fine for elemental melee builds, but you'll probably have to use quarterstaves instead of maces because all the mace skills that are gonna be in Early Access are physical or fire themed, while Sorceress needs other elements to get full use out of her ascendancy.

Also keep in mind Huntress hasn't been added yet but she will be a spear wielding class (you can, of course, use any weapon type on any class but she's themed around spears) so that might kinda be closer to the fantasy you mean.