I do like this idea, and it seems reasonable. Really, I would like to see anyone (human or beast) take a shotgun blast to the face with their back against the wall, and say it is not devastating.
I was thinking something along these lines the other day, pertaining to the double shotgun. The double shotty usually knocks back twice, and has rather sudden damage dropoff, which in certain situations can lead to the damage to decrease. For instance, you shoot with the double shotgun and knock a baron back two squares; the second round will be further away, leading to less damage, even though the shots should have been at the exact same time. In reality the double-barreled shotgun fires one big blast. The double shotgun could be changed to one big blast, but would likely be too powerful in that case, and would need to be weakened in even more areas to compensate. I would prefer to see this situation included, since enemies and the player can bounce off of walls from knockback, and these walls shouldn't be made of soft rubber to bounce off. Knockback into walls should cause some damage, based on the force of the knockback. Perhaps even a knockback-stun could be included on enemies. I would say that enemy collisions should have a very small damage amount (1-2 damage), or the stun situation when enemies fall upon each other. Each enemy could have a chance to be stunned, with bigger enemies being more resistant, and perhaps mancubi would be invulnerable to stun due to their size, same with cyberdemon.
The melee push idea would be ok if there were uniques or exotics like hammers. Pushing could not be done with knives or the chainsaw, as far as I can see, and push should not be that strong for simply fists. I would say that the Push numbers could be decreased, since 2 squares minimum is quite large, and 10 squares is a very long distance for most maps. The problem is, why push when you can punch? I can see some uses (clearing a doorway of a baron, pushing into lava) but not enough to work it into most melee weapons. I think the Push idea would be better suited for a new melee Unique hammer (my vote for Thor's Hammer!)