I agree that 2-hander is the most damaging, but having the bonus action attack lets you spread the damage out to different opponents and when you can move between attacks that is pretty helpful. The shield master feat increases your and some of your allies damage output by letting you shove an enemy prone and getting advantage on attacks against it, it also helps against spells and effects. We also don't know how they are going to handle magic shields yet, if they give a bonus to AC just like armor at higher levels having a shield will be the only option to have ridiculously high AC.
I am not a math guy so not sure how all of this breaks down but, I think a well balanced group should have a mix of types a shield users to be on the front lines, a 2-hander for single target damage, and a dual wielder to take care of minions and stragglers.
Can you point out the part of Great Weapon Mastery that says you can't move during attacks? We couldn't find it. The feat says if you crit or bring opponent below zero, you get attack as a bonus action attack. Doesn't say you can't move before you take it.
I accounted for Shields providing a +1 bonus to AC. If shields and armor provide more than a +1 bonus the game will be slanted so far in the favor of the players as to make even the highest level encounters trivial. A Challenge 23 Solar only has +15 to hit. If they allow more than +1 on Armor, your AC potential becomes in excess of 24. AC 18 plate, +2 shield, +1 defensive fighting, +1 ring, +1 armor, +1 shield. AC 24. Solar needs a 9 or better to hit. This is one of the toughest creatures in the game. That would mean almost everything else would have a far more difficult time hitting. Given hit points and healing, you would make encounters trivial on the part of the PCs. I thought they were trying to avoid these problems. You would also negate the effectiveness of lower level creatures still providing a challenge. That's why I figure magic bonuses to AC wont' go above +1.
I imagine magic weapon enhancements won't go above +1. Otherwise you make boundless accuracy sort of pointless. Though hard to be sure given magic weapon spell maxes at +3. I figure that is for a maximum of an hour and requires a limited spell slot.