No offense but why do you need to design a new forum software? There is a lot of options out there (Phpbb, IPB, vBulletin...). You might want something different than all these guys but any of those can be modded the way you want them. I had phpbb before and I changed the design of the board myself it's pretty straight forward. You can even add some functionalities yourself. To me it just seems like a waste of time to design from scratch a forum software that is secure, fast, reliable, and more...