To start with, I’ve been in active D&D groups of every edition sense 2nd and can not recall a time with no gaming group prior to the launch to 4E and the divide it created in my group.
I personally liked 4E in many ways, but never the less it broke my long standing group apart along strict party lines.
To be fair it wasn’t 4E that really began to erode my group, it was just the excuse many needed to exit what was already a slow to change with the times beast.
The major issue with D&D regardless of edition (and table top gaming in general) is in the nature of modern social interaction, it’s just too difficult to get 4-8 busy individuals together in the same physical location for 4+ hours every week.
4E had some grand plans to fix this, but they failed for 2 important reasons; 1: The virtual assets cost money. No paid system will ever work for the virtual assets because every one in the group including new players must have access to them at all times. 2: The virtual Gametable was never really finished, as the most critical part that was bad.
It’s my simple stance that for tabletop gaming to succeed in the modern era an easy to use and highly modable set of virtual tools must be made freely available on as many platforms as possible, the cost of which is easily justified by the continued ability to sell and develop gaming to new generations.
As for the actual changes to D&D as a rules set, I found it always worked best when at it’s most modular (ie. AD&D 2nd Ed. With Skills&Powers and D&D 3.5). I think the skeleton of modular mechanics should be supplied in bulk and in as few sources as possible, which is to say one good sized rules book with the mechanics to build and the usual verity of supplemental books to provide skins with which to cover said skeleton. Example: The core PHB gives all the rules to craft a PC that uses unarmed combat and employs a bite attack as well as a rage mechanic, additional supplements give details on how to flush out the mechanics to be a half werewolf or perhaps in another a half vampire. World specific supplements (or flavor books as I like to think of them) could provide details on how said build would fit in that world.
In short, give the players all they need of the tools to craft what ever character type they can imagine all in one book, while freeing up the optional flavor from the base mechanics and imparting it in flavor books. All crunch in one and all fluff in others.
The advantage to this is simplicity, keeping the rules all together in a single stable book instead of spread out across a sea of supplements.
As a GM I wasn’t to be able to tell new players all they need to make what ever character they can imagine can be found in a single book purchase, with all others serving as aid to world building and/or imagination. And I want to be able to point them to a free table app we can all use to employ the new gaming rules with.
To sum up: Single modular rules character construction book (something along the lines of what D20 Modern had stumbled on to). Other books contain non core rules and character decorating fluff. A free virtual gaming table tool.