I believe we already had piercing/crushing/slashing resistances in Divinity, but it just wasn't exposed to the player. I could be wrong.
Imbuing weapons with elemental dmg: yes indeed!
Flanking, backstabbing: yes, and we're trying to get more typical turnbased stuff in. Like indeed, weapon range and knockback and shield bash. We're trying to make sure weapons have tradeoffs (e.g. two handed swords do more damage but take more action points; daggers don't do a lot of damage and you have to get close to your enemy, but take less action points and have a higher chance to do critical damage, and can be used to backstab without the enemy seeing you).
I think you make a very good point when you say that many different types of weapons don't make sense if you can't find trade-offs like these. We are bound to a system that we want to keep simple yet deep, so within that system, it's often hard to imagine, for instance, the in-game difference between a crossbow and a normal bow. Plus, you sometimes have to adhere to unwritten RPG-rules that don't always make sense
