Firstly I must say, I've very excited about playing on this PW. I love Greyhawk, and what you've done so far looks great.
I'd say leave them out in the Alpha release, once you've got an established player base, you be able to see whether it's really necessary to have them. The 20 base clases give more than enough options, also in my experience in playing on low level NWN PW's (City of Arabel mainly) I found they aren't really necesary to the character. As it's more about RP, plots, and interaction. How you choose portray/RP your character isn't limited by your character class choices IMO