The reason why it hasn't happened is because nobody has even tried.
Yes and why have they not tried? From what I've read around in articles and such, I think it comes down to devs not having the confidence or will to take on such a massive project.
The game had about as close to a perfect mixture of FPS, tactical stealth hacking dialogue laden RPG hybrid that could be made. It also was a greater resultant system then the sum of its parts that played out so perfectly that seems to be so elusive to duplicate or build upon.
I think the problem lies in the fact that most devs are experts in only one or two at the most aspects of game design. You'll have an expert in FPS gameplay, and most likely someone who is an expert at presentation, but they'll hire out/sub a story writer. Or, you the best you'll do is get a group of expert designers in the FPS/RPG arena, basically like Mass Effect or Fallout 3 for example. But considering their enormous budget and large teams, you still don't have experts in stealth, hacking, and in part dialogue based gameplay in those games.
I think finding a really good group of expert, foreward thinking, and extremely talented (because basic talent won't produce legendary games) designers to:
1. All work in the same place
2. All work on the same project
3. All build a consensus on the design goals
4. All get along well enough to get a project done in a timely manner
It has to be shown that there are not thousands of game designers with the talent, genuis, and efficiency to make games like Deus Ex.
In fact I think its the rarest type of designer and especially like minded group of designers, in the game industry. It would take a studio like Looking Glass, and actual think tank of talne tand imagination like that to nurture and raise designers in a stable environment of growth and opportunity to do similar projects on a regular basis like Deus Ex.
It is no coincidence that Ultima Underworld, Thief, System Shock, Deus Ex were all linked together in their environment and conception by a linked group of people.
I think Iron Tower Studios, Kerebos, and Stardock have the same type of potential, even though they ar enot doing it in the !st person immersive context. Maybe Scars of War will be the one game that emulates the best aspects of Deus Ex, while stretching its boundaries as Gareth suggests.
I also think Starbreeze Studios Chronicles of Riddick series has some of the inspiration from Deus Ex and System Shock. If they would be given the budget and resources they would be a good candidate to take on a Deus Ex gameplay project, and even step it up in some areas. Maybe they need some inspiration...On their forums or through someone with a contact their haha.
I would really like to see a huge petition signed by 100,000 or more people with a list of the most important gameplay feature requests presented to the Deus Ex 3 devs to show consumer buying power behind the fanbase' requests. If the devs can be shown the green invilved, not just some random forum posts they might take notice, and any influence for the original Deus Ex 1's gameplay is much better then none.