I have been thinking about the suspension issue a lot recently. Part of me thinks it's silly to not have it. I mean, you wouldn't find many other modern wheeled vehicles without. It make a lot of sense from a comfort and control standpoint.
But I can also see the disadvantages, complexity being the big one.
I have actually been doing a bit of research into simply suspension designs and am lusting over a YBB frame at the moment. The designs seems to have a lot of advantage, with cost being the only downside. I also like the Willits B2B design, but haven't herd much feedback. Thudbuster can also be a nice add on, if you not to sensitive to fit issues.
The front seems a little easier. Most coil forks, in my experience, will tolerate a fair amount of neglect. Big tires also work well. The over thinker in me has been designing a fork around a Head Shock/Action Tec type design. Small coil and damper that lives in the steerer, cartridge style assembly for easy replacement, Fargo fork lowers. It'll never get made, just nice to dream.