I'm curious as to what might be the avg point improvement to a player over the course of a season? in DIII.

Too many things can influence those stats. WE, playing time, potential, and practice settings. I would advise you to pick a good D3 team and click on teams profile. Click on ratings and then scroll down towards the bottom and you can see the points their players have improved over the season and in what categories. This will give you a good idea what to do with your team and what you can expect from players with certain WE and playing times. Practice settings you will just have to guess unless you compare it to your own when you look at formation ratings season change as well.

Jhunterz gives you good advice. The breakdown of the dif factors are WE is how hard the player will work to get better.
Playing time = the more he plays his WE grows.
Potential is how high his attributes can get to if all attributes were maxed. Most players will never reach that .
And the practice settings determines how much time he is alloted learning the dif formations

Video #9 from the tutorials goes into quite a bit of what those two great coaches touched on.