2: Shotguns are not weak. If you make them anymore powerful, the double shotgun will basically be a BFG with a hundred shots worth of ammo.
If they are not weak then why there are so few shotgun YAVPs? Why everybody are using chainguns, when they say that they liked shotguns in original DooM more?
3: Shotguns are already weak enough at distance that it can be a tough choice between them and chainguns. Make their damage scale anymore, and you'll have to carry both around. I don't know about you, but there's not enough room in my backpack for that.
I'm inclining to original DooM concept: shotty there fires some king of solid slugs(it have scattering, but not so bad, so it hits pretty well on long distances), and deals approximately same damage on both short and long distances, while double shotty fires lead, so it's only useful on short distance, but it deals REALLY great damage. And they both are using the same ammo. Combat shotty could be as it is now, anyway after appearence of Shottyman it don't have any advantages(except 4 more shells that you could carry in it)). As the result shotgun player should carry both shotty and double shotty, first to deal on long distance, and second to turn around corners.))