Well I don't know about how the contract works on hentai game company there, but I think it's very hard to collabs with great writer that work on other companies. Writer on H-game company is really a very valuable assets, second only to main CG /character artists. Unless the writer is freelance/not bind by contract or there's a formal collabs between two company then I think it's very hard to outsource work from famous writer, because most of famous writer already recruited by company and bind by exclusivity contract. Even on doujin circle (not registered as company), when one circle want to get help from other circle's artist, those circle are formally announcing their collabs, not just circle "a" announcing they collabs with artist z (from circle "b") => recent case in point Dragon Blood game collabs. So in the end I think it's all about money and exclusivity.
For Illusion they learned their lesson from A-GA and never attempt to focus on story heavy game anymore. Every game after A-GA is becoming lighter and lighter in story and focused more and more on simulation and interactive 3D .
But even if, hypothetically, Illusion can snatch some famous writer, it would be a titanic undertaking because, as you know already, story heavy hentai games consist of tens of thousand of spoken lines, and unlike 2D games when you just draw a few frames of characters mouth to make it speak (or not even bothered with it, just draw static frame, which is like 90% of VN games out there). In 3D games you HAVE TO animate every words, and change the face expression depends on context of what is said, because if you don't do it then the 3D will become meaningless. And because of that such game will be in development hell for years and years (because at that time 3D game is still in early year, and you have to manually animate everything by hand) and Illusion just can't afford that. That's why even if Illusion can pay some famous writer, they CAN'T pay for the whole time the game in development.
Even TODAY, with so many advancement in 3D tech, including full body mocap and face mocap (and has become very cheap to boot), there's NO story heavy 3D hentai game produced. Why? because even with all those advancement, it's still need a big investment to produce such game. The kind of money that no 3D hentai game company can provide, plain and simple. So yeah, if you want a good story heavy games you still have to look at 2D VN games. It may change in the future, maybe some of those AI might help, but right now it's still just a dream...