Everyone here seems to think they are a better game designer than Mearls and Co. I do not think I am for example but I think I can get something closer to what my group prefers than what D&DN is turning out to be. Designing your own RPG is going to be a huge amount of work so I would borrow idea I thought were done well in other game systems. I would also want to be able to use material from other game systems such as the 2nd ed Encyclopedia Magica and get the numbers close enough where one can use Pathfinder and 4th ed monsters and maybe just tweaking the ACs and hit points. Close enough is good enough- it doesn't have to be perfect.
As to the balance brigade you will never get it right. A meteor swarm for example might deal a bucket load of damage but whoever gets clobbered by it may have a ring of fire protection or a frostbrand sword. Direct damage spells were never broken in 3.5 ed but one would what to tone down the save or suck from that edition.
Anyway I would focus on the major problems and advantages of the editions.
1st Ed: Simple
2nd ed Feel, multiple play styles supported
3rd ed: d20 mechanics
4th ed: Balance
1st Ed: Mechanics
2nd Ed: Counter intuitive mechanics
3rd Ed: Balance/CodZilla
4th: Missed the classic feel.
Try not to get to upset about the pro and con list above, I prefer to look at the larger picture than obsess over what version had the best fighter or whatever. The goal would be to design a modern version of D&D that may not appeal to everyone but hopefully make it appeal to enough people. You will not get everyone on board and the hard core grognards and edition warriors you will not get regardless so one basically ignores them.
d20 Mechanics seem popular enough, retroclones are using them and whatever problems 3rd and 4th had I think the basic d20 was a good one. That may or may not include feats but more the roll d20 and roll high. One also wants something a bit easier than 3rd and 4th ed skills. Personally I would yoink the Pathfinder skill list, throw out class skills, and yoink a modified version of Myth and Magics skill system.
Balance. Put simply I would ignore the balance brigade and go for feel over mechanics. This does not mean I would want a return of Codzilla but more like the tier 3 classes of 3.5 would be my goal post. The 3.5 era spellcasters would get hit hard with the nerf stick. No more natural spell, broken spells would be gone or rewritten, and classics like fireball and magic missile would return to the classic versions of them. They were not broken in 3rd ed due to hit point inflation and other changes made from 2nd ed. The numbers would also be overhauled with numbers closer to 2nd ed than 3rd and 4th with attack numbers between classes like rogues and warriors of course being narrowed. Probably just recycle the numbers form SWSE/3.5 here. Saving throws would be fixed at DC 20 and 2nd ed style save progressions would be used. High level PCs would only have a 5-15% chance or so of flunking a save. That would also be mitigated via feats/ or powers that would allow rerolls as well similar to Star Wars Saga and 4th eds reroll mechanics. If one gets nailed by Finger of Death and you die oh well that is what raise dead is for.
Modules and 4th ed. So far 4th ed has more or less being thrown under the bus. And then reversed over. Since my version of D&DN is more or less 3.5 era d20 based it is not hard to use 4th ed ideas in it. Hell I would be using 4th eds action resolution (minor/move/standard) and reaction mechanics would be rare. The races would have floating attributes similar to essentials, and some mechanics would be optional. Elven accuracy for example could be the AD&D +1 to hit with swords/bows or the 4th ed racial power. Take your pick I do not care. My fighter would resemble a SWSE soldier with 4 skills and talent, feat, talent, feat type progression. Rogues and Warlords would have something similar with inspirational and tactical talent trees used- think noble and officer talents from SWSE. Powers would also be present and they would be unlocked with feats or talents. I won't lie and claim I could match 4th eds system but you would have a list you can pick from. If nothing else it is a gesture of good faith and one can always add more later like the SWSE splats.
Other classes of course could access them so a D&DN fighter using MC dabbler feats similar to 4th ed could pick up magical powers so a high level fighter dabbling in arcane powers might be able to zap a wizard with lightning bolt flying out of his sword. A mundane fighter would have access to vorpal strikes, be able to lop of limbs. An ultra high level one may be touched by the gods and be able to jump to the moon or split a mountain although one may have to use something similar to destiny points and the force unleashed feat in SWSE to do things like that. Things like that would be optional though so old school DMs can say no if they like. Mundane martial types would also have options letting them interfere with spell casters as well so a level 20 wizards vs a level 20 fighter the wizard may have trouble resolving spells.
So thats more or less it. d20 based game using elements from all editions of D&D. I would not bother trying to appeal to everyone, you would have your warlord and dragonborn by now, and class varients can always come later. Balance would be tier 3 3.5 classes mixed with parts of 2nd ed and it would be up to the DM to sort out the details based on what type of game they are planning on running. Saving throws would be higher than what they were in 3.5, broken spells like 3.0 haste, timestop, and polymorph in general would not exist and I would aso use D&DN concentration mechanic.
All sorts of badwrongfun though.
Fear is the Mind Killer