Alignment - Bring back the Law-Chaos, Good-Evil axis. It was just more fun, though "True Neutral" should probably be describes more like Unaligned.
Classes - Reach back to AD&D for the Original Super-Classes (Fighter, Cleric, Wizard, Rogue), and then give each one a classical default Sub-Class with a traditional role and 2 alternative Sub-Classes with alternative roles.
For example:
Super-Class: Fighter
Sub Class Default: Slayer (Striker)
Sub Class Alt. 1: Knight (Defender)
Sub Class Alt. 2: Warlord (Leader)
Doing that would give you 12 Classes, with great variety.
Feats For Customization - Use Feats to really allow people to differentiate what tricks their build has up their sleeve. Want a Barbarian? Take your Slayer and start him down a path of "Primal" and "Rage" feats. Also, Feat-based Multi-classing needs to be revisited from 4E and made more viable.
Move Daily Powers (and Vancian Magic) into an Optional Rules Module
The game really paces, scales, and plays differently when your character's abilities (rather than their stamina and health) diminish over a day. When this option is chosen, ALL CLASSES have properly balance and scaling Daily Abilities they can use and manage. In the core rules, however, everything is either At-Will or Encounter based except for Healing Surges.
Rename the "Healing Surge"
"Heroic Reserves" sounds better. The idea of using Surges as a commodity has cropped up in 4E products recently, and it makes good sense. Give "Squishy" characters more ways of burning their surges other than throwing themselves in front of buses or have bad front-line fighters.
Keep Monsters and Characters Separate
4E Had this right. Monsters are mostly one-shots, props, plot-devices, and villains. Characters are for keeps. Don't put them on the same scale.
Improved Skill Development
3E was much too fine-grain both in terms of how many skills their were and how many levels you could advance in them. 4E was pretty close to the mark in terms of Skill Categories, but way too ham-fisted in terms of Skill Levels: Untrained (+0), Trained (+5), and Expert (+8)? We need something in between, and with options as to whether to continue advancing in one field or acquire another. Just because I used to Intimidate in my callow youth doesn't mean that by Epic Tier my Intimidate skill is nigh-god-like but I've never had a chance to learn Tracking or something similar.
Save or Die Failed It's Save - Let It Stay Dead
I don't want to see Baleful Polymorph or Finger of Death type effects being dropped on PCs. Maybe if they only targeted bloodied monsters and had reduced effects (HP damage and stun / daze) vs. elite / solo monsters they'd be appropriate Character abilities for mopping up otherwise dragging combat.
Nobody is Obsolete
Swiss-army-knife Magic is the bane of skill-based characters everywhere. Magic has to walk a fine line between pulling the proverbial rat out of your hat at the dramatic moment and making a skill-monkey just a sack of HP that's largely redundant up against a good spell-book.
Grids are Optional
I like minis and map-based tactics, but some people don't. Figure out a way to present basic ideas like "Flanking" and "Opportunity Attacks" into environments that don't rely on grid-lines.
- Marty Lund
Classes - Reach back to AD&D for the Original Super-Classes (Fighter, Cleric, Wizard, Rogue), and then give each one a classical default Sub-Class with a traditional role and 2 alternative Sub-Classes with alternative roles.
For example:
Super-Class: Fighter
Sub Class Default: Slayer (Striker)
Sub Class Alt. 1: Knight (Defender)
Sub Class Alt. 2: Warlord (Leader)
Doing that would give you 12 Classes, with great variety.
Feats For Customization - Use Feats to really allow people to differentiate what tricks their build has up their sleeve. Want a Barbarian? Take your Slayer and start him down a path of "Primal" and "Rage" feats. Also, Feat-based Multi-classing needs to be revisited from 4E and made more viable.
Move Daily Powers (and Vancian Magic) into an Optional Rules Module
The game really paces, scales, and plays differently when your character's abilities (rather than their stamina and health) diminish over a day. When this option is chosen, ALL CLASSES have properly balance and scaling Daily Abilities they can use and manage. In the core rules, however, everything is either At-Will or Encounter based except for Healing Surges.
Rename the "Healing Surge"
"Heroic Reserves" sounds better. The idea of using Surges as a commodity has cropped up in 4E products recently, and it makes good sense. Give "Squishy" characters more ways of burning their surges other than throwing themselves in front of buses or have bad front-line fighters.

Keep Monsters and Characters Separate
4E Had this right. Monsters are mostly one-shots, props, plot-devices, and villains. Characters are for keeps. Don't put them on the same scale.
Improved Skill Development
3E was much too fine-grain both in terms of how many skills their were and how many levels you could advance in them. 4E was pretty close to the mark in terms of Skill Categories, but way too ham-fisted in terms of Skill Levels: Untrained (+0), Trained (+5), and Expert (+8)? We need something in between, and with options as to whether to continue advancing in one field or acquire another. Just because I used to Intimidate in my callow youth doesn't mean that by Epic Tier my Intimidate skill is nigh-god-like but I've never had a chance to learn Tracking or something similar.
Save or Die Failed It's Save - Let It Stay Dead
I don't want to see Baleful Polymorph or Finger of Death type effects being dropped on PCs. Maybe if they only targeted bloodied monsters and had reduced effects (HP damage and stun / daze) vs. elite / solo monsters they'd be appropriate Character abilities for mopping up otherwise dragging combat.
Nobody is Obsolete
Swiss-army-knife Magic is the bane of skill-based characters everywhere. Magic has to walk a fine line between pulling the proverbial rat out of your hat at the dramatic moment and making a skill-monkey just a sack of HP that's largely redundant up against a good spell-book.
Grids are Optional
I like minis and map-based tactics, but some people don't. Figure out a way to present basic ideas like "Flanking" and "Opportunity Attacks" into environments that don't rely on grid-lines.
- Marty Lund