upcoming 5th edition? Over the past year, Monte Cook and Mike Mearls have both voiced a strong desire to make D&D into a modular design that individual DM's could use to better create the game that they want to run. The previous edition that comes closest to this ideal is 2nd and probably not coincidentally, Monte Cook and Bruce Cordell were both developers at TSR during the days of 2nd.
Why is 2nd edition the most modular of the editions? Just look at the core rules. They are loaded with side bars with optional rules for damn near everything. Every class outside the main four are optional. There are six different ways to roll stats. Three different ways to do initiative. Two different ways to do critical hits (and those are optional as well). Options were everywhere and allowed dm's to run fairly different games of D&D right from the beginning.
Now look at the slew of optional rule books that came out for 2nd. Yeah, yeah, I know - 2nd edition created the word "splat book", but no other edition has had so much official (non-3rd party) support and options to chose from. From the horde of class and race books to rule books for creating castles or running campaigns at sea, 2nd edition had you covered. By the end, it also had the Player's Options books which allowed DM's to even more radically change the game they were running. 2nd edition had options and modular systems out the wazoo. Yeah, a lot of it was unbalanced and poorly thought out garbage, but the concept was right and I think this is the path they are looking to follow once again.
Time will tell. I'm signed up for the playtests. It should be a very interesting year!