Generally speaking when I think about the next generation of DnD I think that 4th edition could use some refinements and most of 3rd edition can be safely tossed. There are some really unique things that 3rd had though that I often wish could make it into the new game. The biggest one is the inherent flexibility of the magic system. In 4th nearly every ability is a strictly combat ability. Even abilities that are called utility powers are framed around usage in combat and often have no applicability outside it. There are some exceptions of course, as some utility encounter powers teleport the caster or let them fly for a short distance and these could obviously be utilized outside of a fight. 3rd was very different because casters had to choose to allocate limited resources to combat or noncombat applications. I could memorize Levitate or Melf's Acid Arrow, Fly or Fireball, Dimension Door or Ice Storm. In each case there is the possibility of using each spell in the opposite arena but you honestly don't need to Fireball the forest or Levitate in combat very often. Even if I am a Sorcerer and don't memorize spells I still have a very limited number of spells known and I really have a tough choice between utility and combat spells.
I think people really miss this in 4th because everything is so cleanly divided. All of your class functions are combat abilities that *might* have an application outside combat but usually not. When myself and some friends were designing a RPG system years ago we encountered this problem - we could never balance classes by giving some more effective noncombat abilities and some more effective combat abilities and have it work out right. We ended up deciding that characters need to all be effective in combat and all have some interesting things to do outside of combat though those things could run the gamut from thieving skills to utility spells to social powers. 4th really does leave the player who wants to have lots of utility and creative abilities out in the cold as rituals just don't cut it for that purpose. They have their place, and work fine for what they do, but they just don't do everything people want.
I don't know if there is actually any good solution to this problem. If you give people real flexibility in character design like in Mage or Vampire for example you end up with some people who utterly dominate combat, some who dominate social interactions and some who dominate in other ways. If you want a balanced system where you can have a drop in gaming group and not be worried about hideous balance problems from a new player you have to make things extremely regimented (like 4th does) and disallow tradeoffs between combat and noncombat prowess. 3rd, on the other hand, does allow this sort of tradeoff and thus the players have a ton of flexibility. However, if you have a new player come into your campaign you may well have someone who is utterly ineffective against the monsters your group comes up against or someone who can handily defeat all the encounters by themselves. In my current gaming group I had to ditch a character who was simply too overpowered for the group and do something else because I was making things unfun for the group - this is *really* tough to do in 4th and utterly trivial in 3rd; just play a wizard, druid or cleric!
Old school DnD players are used to flexibility and having noncombat and combat abilities be defined by their player's imaginations and creativity. 4th edition does away with that in the name of balance and I find myself longing for the fun choices of the past. I don't find myself longing for the days of rogues watching on the sidelines as the wizard defeated the campaign by himself or fighters having nothing to do but "I attack." though. Whether or not both objectives can be achieved simultaneously is an open question but my suspicion is that they cannot.