I'm having a blast designing alternate classes, class archetypes, new traits, new feats, new spells, new equipment, for homebrews, and for my
Kaidan: a Japanese Ghost Story setting for PFRPG.
I use the CRB, APG, UM, UC and GMG, and my own ideas as well as the experience I've gained working with Rite Publishing, Jonathan McAnulty, Will Cardill, Trevor H Gulliver in the creation of my Kaidan setting for publication - I've learned a lot from these people.
I don't think Pathfinder needs some kind of add-on system to do this. The basic game is made to be customizeable and I've been creating custom classes since PF first came out. I find it easier to design new classes and class mechanics than even 3.5 - Pathfinder is better for customization.
First I started playing with archetypes with Ranger which seemed an easily customized class. Afterward I've since tried to build archetypes for every single class - even one's I didn't care to run in the beginning, now I've touched upon every single class. I've become fans of classes I never were in the beginning with the values of customization.
I don't need an add-on system, the existing system works fine right now.