The proficiency damage is a good idea.
I'm going to do a level based damaged increase:
Lv 5 = +1w
Lv 9 = +2w
Lv 13 = +3w
Lv 17 = +4w
And the magic item gives the old +1/+1, so when you find a +5 sword at lv 17 you will have 5w (base + level damage)+ 5 damage (item bonus) that is similar to the 6w you should do with normal rules (the averange damage is more or less the same).
I wish that in the final version, they adjust the proficiency of all classes and use that to add damage to the weapons.