I would not necessarily say that D&D needs "fixed," but I think that a modular version of D&D could benefit the hobby. I would actually look to games like 4e and Shadow of the Demon Lord about how to potentially approach this. 4e recognized tiers of play at work, and SotDL expanded on this in its adventure and class design. Then I would take a cue from video games with easy, standard, and hard modes.
The game could be broken up into Tiers of Play: Basic (levels 1-5); Expert (levels 6-10); Advanced (levels 11-15); and Epic (levels 16-20). The game could be played from 1-20 or you could pick the game up at a later tier. Each tier adds additional options and features.
Adventures and Modules would be written for a given Tier of play rather than for set levels of play (e.g., Expert adventure rather than rapidly shooting characters through levels 1-12).
Monsters would likewise be designed for tiers of play, with DM instructions for upgrading monsters to different tier levels or encounters. Here though I would also bring back more 4e-style monster design. But additional options could be added for morale, bloodied, and/or optionally minion rules.
Classes would also be designed with this modular Tier system in mind. Maybe you start out as one of four basic classes (i.e., Fighter, Priest, Rogue, Mage), but then when you reach Expert level you choose an Expert class (e.g., Bard, Cleric, Ranger, Paladin, Druid, Psion, Marshal, Warlock, Wizard, etc.) or even a second Basic class. Likewise you later choose an Advanced Class and an Epic Class once you reach those tiers of play. Maybe even make it so, if you want, you could go from Warrior -> Paladin, Priest -> Paladin, Rogue -> Paladin, or even Mage -> Paladin. Build that character you want.
Layered into this Tier system Beginner, Standard, and Veteran modes could be provided for the Game Master. For example, Beginner Mode may provide Class HD/level + Con Modifier/level to HP, while Average cuts off Con Mod/level after Basic/Expert tier, and Veteran mode removes +Con mod to HP per level entirely. But have these BSV rules labeled and organized as such.
I think that this would also work well for Organized Play, because you could set up various tables for adventures and say, "This is an Advanced level adventure set on Easy mode at this table, the same Advanced level adventure set on Standard mode at this other table, and a Basic level adventure but set on Veteran mode at this table,..."