I'm going to go with adz000 and psilord here. There are already several acceptable Greek textbooks available, both free and currently in print.
I'm going to go out on a limb, and assert that you don't actually learn Greek (or Latin) from beginning textbooks. Rather, these books prepare the mind with the basic mechanics of the language, the technical vocabulary used to talk about language (accusative of respect, result clause, contraction, etc.). Plenty of people have learned this - they go through a textkbook, and then wonder what language they learned once they hit a real Greek text.
You learn to read Greek by reading Greek, as much of it as possible, with the help of a good commentary for when the rules stated so clearly in the beginning books fall flat on their faces.
So I don't think the English speaking world needs a new Greek textbook right now.
Now, having said that, I have often fantasized about taking one or two of the Textkit beginning textbooks and turning them into regular web pages, in particular Pharr. This would be a massive data entry project, requiring people to type pages as well as people to check the work. Once that was done we could go in and add extra comments, fix up obsolete or incorrect philology, etc.