Well, I think I mentioned what I'd like to see in previous incarnations of this topic, though I think a few of my original ideas may have changed.
What I'd like to see:
Fewer core classes. Ideally, I think there should only be a few core classes: either 1 core class per each attribute (Str, Dex, Con, Int, Wis, Cha), or just 3 core classes that cover the basics: combat & feat-focused, special ability & skill-focused, and magic/power-using focused. Each class should have a few set class skills, and the rest should be selectable (much as it is with the Generic Classes in UA).
Personally, I'd go for the 3-core class variant, esp. with the option to use or not use different categories of magic/power (arcane magic, divine magic, & psionic power, for example).
Go the d20 Modern route and use Talent trees to allow characters to select the style of character they want to play. Also, none of these core classes should have Alignment restrictions--that should just be a feature found with PrCs.
Change some core classes to prestige classes. Specific class concepts like knights, monks, paladins, druids, (the armed & armored) clerics, soulknives, beguilers, dread necromancers, rangers, spellcasting bards, samurai, & the like should be prestige classes instead of core classes. Any class with a "fixed" theme, certain requirements, or the like should be a PrC option. However, have the limited core classes still be viable selections on their own (unlike the required multiclassing of d20 Modern).
More "variable" racial abilities. Elements of a PC race that are based mainly on biology (like ability score modifiers, special vision or sense-based skill bonuses, etc.) should remain as is.
Culturally-based racial abilities, like the elves' free proficiencies with rapiers, longswords, and bows, or the dwarves' bonuses to Craft and Appraise, should be selectable. Allow for some degree of choice by letting a player select a culturally-based bonus feat from a limited list for certain races. That way, different groups/cultures of the same PC race can be created with the same ruleset instead of having to make a new variant or subrace of an existing race to account for them.
Fuse some skills together. This has occured to some degree in SWSE. A general "Perception" skill instead of seperate Listen & Spot checks. A general "Stealth" skill instead of Hide and Move Silently. A general "Spellcraft" skill instead of Knowledge (arcana) and Spellcraft (and maybe even Use Magic Device). Some feats could be dropped, since an appropriate Skill Focus feat could do the same job (or even slightly better now).
Make a more "generic" spell list that's customizable. The main example that comes to mind is combat spells. Instead of having specific spells like fireball, lightning bolt, and magic missile, have all spells deal force damage, an area of effect and certain conditions to hit (attack, touch attack, auto hit & save for half, or auto hit w/ no save).
If the character wants to have the spell deal a specific type of energy damage other than force, then add the energy, then increase the damage die of the spell (say from a d4 to a d6), have it gain an additional benefit for the different energy type (like fire lighting flammable materials, or acid doing additional damage the following round), but it also has energy-related limits (creatures with resistance or immunity to the energy type, not able to use the energy type in certain environments, etc.).
Provide generic versions of some monsters that can be easily modified as core. Have a "blank template" version of a monster that can have styles or themes added to them (esp. for monsters than have a lot of varieties based off of them, like giants, dragons, celestials, fiends, & the like).
For example, a generic Dragon base monster, which can have color-based themes added to it (such as simply adding a "red dragon" or "blue dragon" or "silver dragon" or "amethyst dragon" or "shou lung dragon" template to the existing core Dragon monster). This could allow DMs to simply recreate the existing D&D varieties of dragons, create new ones of their own, or even replace the existing ones and use their own campaign-based defaults (like having only 5 different types of dragons based on the different elemental energies, for example).
Take a new approach with Exotic Weapons. There seem to be few Exotic weapons that seem worth the Exotic Weapon Proficiency feat. I'd say to make this feat seem worthwhile, some other types of weapons could go into this category, or possibly gain an added benefit if this feat is taken. Two examples:
Make composite bows Exotic weapons. The benefits are increased range, increased usability (such as on horseback in some cases), the option for stronger bows that add in Str bonus to damage, etc.
-or-
Give Exotic twists to some martial and simple weapons, which need the EWP feat to be used by a character. Allow rapiers and scimitars to be used as Finesseable weapons or special monk weapons with an EWP feat. Allow longswords to used as piercing weapons or short swords as slashing weapons with an EWP feat. Allow battleaxes and warhammers to be throwing weapons using an EWP feat. Let a spear user extend the reach of an attack or use it one-handed using an EWP feat. Let a shield be used to bash without losing the AC bonus or be used as a thrown weapon with an EWP feat.
Well, those are my ideas for a potential 4E.