There are lots of tarp designs that are worth looking at. The question is really what you want to do with the tarp. I have a DIY 12x10 tarp that is really great for spending a rainy day in camp. Not the best choice for fast and light, though. Too many tie-outs to mess with, too heavy, too bulky. A smaller, cat-cut tarp is on my project list, but pretty far down.
You could make a priority list (weight, ease of setup, coverage, versatility for different pitches, wind protection, durability, cost, dual-use, difficulty of construction etc.). Based on what is important to you, it should be pretty easy to figure out the right tarp design. Might be a tarp/poncho for fastpacking, or a 12x10 rectangular tarp for chilling out in camp, or blackbishop's design. The nice thing about making it yourself is that you can get exactly what you want.