The overall theme of my revisions would be to reduce the amount of fiddling with the mechanics. 4E is a great improvement over 3E in this regard, but more could be done. Specifically:
#1: Gridded battlemats should not be the default assumption.
I agree with the overall sentiment on this. It's one area where 4E went in the wrong direction IMO; too many powers and abilities rely on exact knowledge of your character's location. (One idea I've been batting around my head for a while is the idea of a "gridless battlemat," where instead of having a grid, you sketch out a number of areas - main room, balconies, evil altar, alcoves, et cetera - and all you have to keep track of is which area your PC is in.)
Above all, I want to abolish the counting off of squares. The last thing I want players to be doing in the heat of battle is counting squares and measuring distances; I want them busy imagining the battlefield and coming up with crazy brilliant maneuvers. If there is a battlemat, you should need no more than a glance at it to tell what your tactical options are.
This does require a shift in design. A lot of the intricacy of 4E combat comes from those fiddly little battlemat movements. To maintain tactical depth, some other element would have to replace them.
#2: On the scale of "perfect game balance" to "simplicity," shift the game a bit more toward "simplicity."
I'm glad that 4E decided to take a solid, mathematical approach to balancing things, but there are some areas where they went overboard - particularly with magic items. The rules for tracking daily power usage are ridiculously convoluted. They should be streamlined; or, better, do away with magic item daily powers altogether and focus on encounter powers and always-on properties.
Likewise, I recently concluded that there was no good reason why healing potions ought to cost you a healing surge. In fact, healing potions that don't cost a surge would be a nice way for the DM to give the PCs a little extra go-juice for a lengthy series of battles.
#3: Design mechanics with ease of understanding in mind.
Please note that I'm not just talking about ease of
use here. 4E is mostly good about that. I'm talking about ease of
understanding; making the mechanic simple and intuitive, so that it's easy to learn.
In some cases, this is just a matter of presentation. Take healing surges. In my experience, new players often get confused about when they can spend healing surges and how; and I think it's because healing surges are presented as something you "have" and "spend," implying that you can spend them any time you like. Instead, suppose they were presented as follows:
Certain effects can inspire you. When you are inspired, you can recover hit points equal to one-quarter your maximum; if you do so, however, you lose a point of Stamina. Your maximum Stamina depends on your class and Constitution score. When you run out of Stamina, you fall unconscious until you take an extended rest.
Once per encounter, you can use the Second Wind ability. This ability inspires you and gives you +2 to all defenses until the start of your next turn.
After each extended rest, you recover all lost Stamina.
Mechanically it's identical to the current system, but the presentation makes it easier to grasp. It also provides Stamina as a tool for DMs to address issues like "What happens if I try to stay awake for three days straight?" (Answer: You lose X stamina per day.)
In other cases, the mechanics themselves could be streamlined. Attack bonuses are a good instance of this. I spend an awful lot of time talking other players through, "No, see, your attack bonus is equal to your Strength bonus, plus half your level, plus your proficiency bonus, plus the magic bonus on your weapon... and now your damage is your Strength bonus - no,
don't add half your level - plus the damage die for the weapon, plus the magic bonus... but you're using
this power, so you also add your Charisma bonus..."
If I were redesigning 4E, I would abolish the use of ability score bonuses that don't include half your level. I would also get rid of weapon proficiency bonuses. In general, the default case should be the simplest to calculate; and the default case is that you're proficient with the weapon you're using. The earlier editions had it right on this one. Nonproficiency should be a penalty, proficiency should not be a bonus. If you want points of distinction between different weapons, that's what class powers are for.
(I'd like to abolish magic item "plusses" as well, but that might be too much of a sacred cow. Perhaps the "plusses" could be some sort of encounter power instead; once per encounter, you can add the weapon's bonus to one attack roll. Likewise, once per encounter, you can add your magic armor's bonus to your AC.)