My envisioning of 4e would be a large rules overhaul.
Base attack bonus is good, so we'll also have a base magic bonus. Only magical classes actually give you MP, though. For the sake of fun, a fighter 19/wizard 1 ought to be able to cast at least 4th level spells, though maybe just one a day, instead of being limited to practically useless 1st level spells. Or heck, maybe use this as a system for monks to use their ki abilities, and sword sages to use their supernatural weapon techniques.
One action = one attack.
Make magic items unncessary for balanced gameplay. I would love it if magic items were rare, and even high-level characters had only one or two magic items.
To go with that, get rid of enhancement bonuses for magic items. I think numeric bonuses are simply boring. A magic sword should be able to shoot fire, or cut through steel with ease, or deflect attacks, or deal bonus damage against dragons. A +1 sword would be a masterwork weapon, but a magic sword doesn't need a bonus to attack and damage rolls. Likewise, if things were balanced better, high-level characters would not need two rings, an amulet, a belt, a cloak, a vest, a pair of boots, a pair of gloves, goggles, bracers, a fanny pack, and a headband.
If you have a belt of giant strength, you don't get a bonus to attack and damage; you get to wield weapons as if you were a large creature, and lift things as if you were larger. So no attack bonus, but your damage dice go up, and you can throw, like, wagons and stuff. But not every high-level fighter would be expected to have a belt of giant strength.
Too drastic?
There should be three basic conditions. Fine, disabled, dying. And I guess dead, but we can ignore that one. Fine is when you've got more than 0 hp. You get a little wounded, but you can ignore the pain. Disabled is from 0 to -9 hp, gives you a -2 penalty to attacks and AC, and limits you to one action per round. If you take two actions, you take a point of damage. Dying is from -10 on down. Each round you make a Fort save to stay alive, and it gets harder the longer you go. When you first fall to -10, you can make a Will save to stay conscious (or you can automatically do this if you have Diehard).
All other conditions would be derivatives of those. Fatigued = disabled, only it doesn't cause damage. Exhausted is like dying, and you have to make Fort saves to avoid collapsing. Sickened is like disabled, nauseated is like dying. And so on.