For instance, here are things I have spotted that, I would love to sit down and take a crack at...
NEW and UPDATED CLASSES:
Barbarian (primal Defender)
Bard (Arcane leader)
Druid (Primal Controller)
Monk (Ki/Psionic Striker)
Psioncist (Ki/Pshionc Controller)
Psychic Warrior (Ki/Psionic Defender)
--If you were to define the ranger as a Primal character and come up with some new class for a Divine controller or striker role, you would then have three classes, covering three distinct roles for every power source, which I think is kind of cool as a start.
MORE ROBUST RULES FOR NON-COMBAT SITUATIONS
--I'm fully onboard with the 'give every character a useful combat role' assumption in the default rules. There is nothign wrong, however, with certain DMs and players wanting to define non-combat roles with stronger mechanics.
Non-combat based power options for each PC class.
Non-combat customization options for NPC's and monsters
LETHALITY RULES
--As above, I like the more abstract and PC friendly healing rules for the D&D default. But what about DM's and players who prefer a more gritty (I hate that term) game, or who want to use the mechanics to create long term injuries and risks.
Optional rules for serious or long-term injuries.
CULTURAL MODULES
-- A combination of feats, powers, races, rituals and classes to carry the core rules into a new style of play
Oriental
Aztec/Inca/Mayan/MesoAmerican
African
Arabian
Classical
Renaissance/early-industrial (clockworks, firearms etc)
SETTING MODIFICATION
-- How do you incorporate early edition settings into a new rule set, what changes / new material is required. Particularly important for settings that streatch traditional fantasy assumptions
Spelljammer (fantasy physics etc)
Ravenloft (Robust horror rules)
Darksum (unique magic rules)
--or in third party settings
Midnight (grim, powereddown)
Oathbound (High action over the top)