Changes I've been considering:
1. Dropping the Weapon Proficiency rules as written, classes are proficient in all allowable weapons.
2. Single class Fighters get the benefits of Specialization for all weapons.
3. Single class Thieves get the benefits of Specialization for the dagger.
4. Multiclass Fighters and Paladins get multiple attacks as if specialized.
5. Rangers get penalty free Two Weapon Fighting, as well as the benefits for weapon specialization(+1 attack, +2 damage, better multiple attacks) with ranged weapons.
6. Other classes get multiple attacks as non-specialized Fighter
7. Character classes either get full BAB as per 3E, or a 2/3 progression similar to the Cleric progression from 2E. There are reasons for this.
8. 4E implements exist as ranged weapons similar to the bow. They perform basic ranged attacks using Int or Wis and can be enchanted like other weapons, with +1 attack/damage for 16Int/Wis, +2 attack/damage for 17-18, and +3 attack/damage for 19+. These attacks target AC. (this is why Wizards get a decent BAB). Must be able to cast spells to use these.
9. Backstab is replaced by 4E sneak attack(x2=2d6, x3=3d6, x4=5d6).
10. Dexterity adds to the damage of Ranged attacks, and Javelins, Handaxes and Throwing Hammers count as 4E Heavy Thrown weapons.
11. 4E movement and positioning, with standard, move, minor actions. Some spells will be minor actions(examples bless, shield).
12. Cure X Wounds spells are minor actions that target an ally within 5.
13. Spontaneous cure spells for Clerics as per 3E.
14. Use 1E spell lists, especially Illusionist and Druid. Bards can choose Mage or Illusionist spells but not both(choose at creation).
15. Strength rules change:
15 str: +0attack/+1damage
16 str: +1attack/+1damage
17 str: +1attack/+2damage
18 str: +2attack/+3damage
19 str: +3attack/+6damage
20+ as normal
16. No racial limits on classes or levels
17. Single class Fighters get 4E Combat Challenge and Superiority
18. Multiclass Fighters get Combat Challenge
19. Paladins get Divine Challenge
20. Use 4E skill system