Should the blade be longer than 3 1/2" ?
If you want to appeal to a mass market, 3.25-3.5 works well for most applications. Most folks that buy are just normal dudes, who carry in a pair of jeans/cargo shorts. Anything longer is not that comfortable. For strictly tactical purposes, 3.75 inch blade is my choice for flight suit or ABU/ACU. The pockets are deeper, so the extra lengh scales don't make any difference. Less of a market for longer blades though...from what I've seen. I carry 4 knives on my person (2 folders and 2 fixed blades), and none have blades longer than 3.5 inches.
Should the spacer be solid or should stand offs be used?
My aircrew folder has a titanium spacer. I like it, because it protects the knife from getting things wedged (coins are bad) in the scales or touching the blade edge. That said, it does take a little cleaning with a Q-tip and can of air once in a while to blow it out. I usually clip the folder to the top flight suit pocket, so it can be deployed easily...and so it doesn't rub on stuff in my pocket. So, it might not be an issue of it doesn't rest in the bottom of the pocket. both have their advantages/disadvantages...I don't think either would keep me from a purchase if I liked the knife.
Are double bolsters acceptable or is this too much weight?
Tough bolsters, then lightweight scales for my taste. If there is a purpose for double bolsters, then fine...but you would have to have a reason. Plus, I don't really like the double bolster look on anything besides a slipjoint.
Are man made materials for scales preffered over natural ones?
Waterproof/sweat proof is very important for a knive that lives in the pocket of deployed personnel. I like micarta, but would not want it on a knife that lives in a soaked flight suit. G10 (many many colors) is tough as hell, light weight, appeals to the tactical market. My aircrew folder has hand checkered scales on the mark side. Some sort of texture can make a big difference in grip. If you want to use wood, then I would suggest using stabilized wood.
Good luck in the market...very competitive. Lots of production models to compete with too. If you can come up with a very usable design and sell it for $250 or less, you might sell volumes. If they go for $300+, you will sell a lot less unless you come up with something very unique.
My nickels worth...