I've resisted doing this, but hey, they're being nice enough to ask what we want, so I'll tell them what I want.
If you're a regular reader here, and don't really care to read this, here's the truncated version: USE THE LABYRINTH LORD MODEL.
Step 1: A single core book (we'll call it Basic D&D, since that's what it should be modeled after) with everything you need to make the standard 4 character types and have adventures. You should be able to roll up characters in 5 minutes or less and get to playing. I would list everything out, but just look at Labyrinth Lord, it has all this. People should be able to buy this book, and have years of adventures with it and nothing else. It should cover levels 1-20 or at a minimum levels 1-10.
Step 2: Produce the books for hacking the core system. See Labyrinth Lord's Advanced Edition Companion and Original Edition Characters books for excellent examples of how this is done. Additional books can be done for a Supers-style D&D, which is basically what 3E/Pathfinder is. These books would include character creation for SRD characters, feats, fort/ref/will saves, the crazy skill system, tactical miniatures combat rules, etc.
Step 3: Make an intro box for people that are new to the hobby. See the Pathfinder Beginner Box for an example of a good one. See James Raggi's Lamentations of the Flame Princess box set for an example of what great referee and tutorial books look like. Inside the box should be a coupon code for a discount on the core book.
Step 4: Monster books should ideally be universal. This one's going to be a challenge, but there are a few things you can do. There should be things that are able to be translated differently depending on how the game is being played. For example, just list hit dice, and we will know that if we are playing original style, then we use d6s, d8s for basic, and d10s for supers-style. Monsters are not PCs, and their stat blocks should be as lean as possible. Perhaps you have the core information, and then a few additional lines highlighted in a different color that are only applicable if you are playing supers-style, or whatever.
Step 5: Now you have the rules done, LEAVE THEM THE FUCK ALONE! If you want to release additional rules supplements, fine, but always maintain the integrity of the core rules. D&D 4E was the only version of D&D where I've ever sold my core books. You invalidated them with a constant stream of errata. We played a 5-month long campaign and almost every session a player came to the table with one of his abilities changed from what it was the previous time we played. You are going to have situations where some players like to use the digital tools to make their characters, and some people are using the books. There should not be discrepancies between the two.
Also, I should be able to play any of my stack of old Basic and AD&D modules with some version of this game. I'd recommend cleaning them up and re-releasing them, at least in digital or print-on-demand form, since a lot of people seem to be clamoring for that, but I'm more concerned about the ability to easily play them, since I already have most of them.
At this point, you have the core rules done, and you will sell some books. Now your business model needs to change. You are not in the business of selling an endless stream of rulebooks. Ideally, the rules should be OGL, and there should be an awesome free SRD site for people to use (see d20pfsrd.com for an example). If you are not going to learn from your mistakes, and you are going to constantly fiddle with the rules so you can sell more rulebooks, you are going to piss people off. Get the rules out there, make them good, and give them away for free on an SRD site, sell them in book form, and sell them in PDF/eBook form (no DRM either!). Then you make money by selling stuff for people to play using those rules (adventures, settings, DM toolkits, etc.). You need to sell creative ideas, interesting and revolutionary new ways to have adventures. Just selling a million new PC build option books is lame. In my home gaming group, nobody buys a damn thing. I'm the DM and I'm the only person that buys stuff, so make stuff for me to use to help me run awesome games for my cheapskate players.
And another thing. Take a look at how you pay people who write adventures and change it. Paying by the word seems to produce some real shitty adventures. Give authors a cut of sales, so they have to actually write good adventures in order to get paid.
Let me tell you a story. When I was still playing 4E, I saw this book called Pathfinder at the game store. I had heard a few things about it on enworld and such, but I was never really a 3E player anyways, and the massive size of the book, it's weird name and weird art made me walk right by. A bit later on, after continuing to hear good things about it, I figured I should check it out, and I downloaded a less than legitimate copy of it. I was so impressed by it, that I am now a subscriber to 5 of their product lines, and have probably spent somewhere in the ballpark of $2,000 in the past 18 months on Paizo/Pathfinder stuff. This is what piracy can do for you. It can give people a reason to buy your stuff. I don't like to buy things blindly. I like to see, with my own eyes, that a product is good. If it is, I will happily fork out the cash for it, and my preferred purchase method is Print+PDF.
Every month, I get beautiful books in the mail from Paizo. Even though I hardly play Pathfinder anymore, their adventure paths are a great value. I get adventure ideas, new monsters, short fiction, and an article on the ecology of a monster or some stuff about a god or something. I can extract the images from the PDF copy and use the maps and pictures of monsters in my games. We have a great relationship. They tell me, "Because you are awesome and subscribe to our adventure path, we'll give you 15% off everything else we sell."
Their campaign setting is also something you should take a look at. They made a campaign setting as a place to play games in. It is not a gimmick to sell novels. It is a highly-usable gaming product, and they come right out and say that it should be hacked and bastardized in the grandest traditions of D&D.
Their novels are not grand epic tales that would turn into failed game sessions. The novels portray normal, crappy adventurers, like actual PCs. They are there to show examples of what kinds of things you could do with Golarion in an actual game.
Digital tools. I think you should have them and I think they should not suck. I think there should be a free component with basic character creation features and the ability to print character sheets that don't suck up $40 worth of ink to print one sheet. If you want to charge a subscription fee for more advanced features, I think that's fine, but they should be tools that enhance my experience and make my life easier. If the game is built in such a way that not having the tools makes me have a shitty experience, you will not have won this customer back.
Some people just want to play with pencil, paper, dice, and imaginations. These people are important to this thing you're trying to do here, so don't take it for granted that everyone's going to be down with needing a computer to make your game work.
So, this turned into way bigger of a wall of text than I set out to write. Sorry! But hey, at least it's all off my chest now. Carry on.