1. Fix your marketing. It was and remains pretty bad ( I have a marketing MBA, btw). This is critical to the success of a new edition.
2. I think you need a default setting. There needs to be a way for new people to enter the game, and w/o this, not sure how they really do it.
3. I think you need more adventures, and better adventures. Same reason as above, plus adventures are what really give us a share experience. Btw, not all of us can make it to a gaming store, so only having the common experiences that way does not help.
4. Something magical is missing in 4E. Maybe I'm just too old, but something is missing. While I love all the powers and options, some of the magic is missing someplace. Also,magic is missing from the magic.
6. I think you need an entry level version of the game. I have no idea how a new player would start right now.
Every game really needs a core focus that all the design choices revolve around. 3E was about codifying everything, to limit the impact of arbitrary GM's. 4E was about balance and simplification. 5E should be about new players, not us old farts.
Along these lines:
1) Fast character creation, without the necessity for detailed builds. You want new players up and running asap.
2) Along those lines, good digital tools, but don't design a system where players/GM's feel they
must have those tools.
3) Simple base rules, with more complex options that can be mix-and-match. Those who like complexity can have it, without burdening all those players who just want to spend a few hours having fun with friends.
4) Limit the math. Computers are great with numbers, most humans do better with concepts and situations.
5) D&D, not Squad Leader. The tactics should be an important element of the game, but not the main focus. My friend's cousin should be able to play the game without feeling useless or like an idiot.
6) Eliminate the need for system mastery. It's okay if someone gains a slight advantage from going through the rules with a fine-tooth comb. But don't make it so that anyone who doesn't ends up with a suboptimal character.
7) Some decent starting adventures, with evocative art. Leave room at the end of these adventures for the GM to add more.
8) I'm less convinced that a default setting is required. But do provide info and examples to GM's on how to create their own. Emphasize the number one rule from the old Dungeonmastering articles - only create what you need, when you need it.
Face it, whatever design decisions are made in 5E, there will be a very vocal group of current or former players who will attack WOTC - they ruined my game, they left out [fill in the blank], they didn't playtest it enough, and so on. Just reading this thread, I see several suggestions that are diametrically opposed. WOTC has to accept that some portion of their current customer base is not going to like 5E.
But I do believe that if they focus on what a new player would want, what would cause someone to choose this game over the literally dozens of other options available, they'll end up with a game that the existing players will also enjoy.