4th Edition Considerations
Before it will be called 4th Edition it will have to be *significantly different* from 3rd edition. New classes and races are cosmetic. Compiling content from the splat/sourcebooks is not "new edition" -- that would simply be a 3.5 Compiled Sourcebook.
I doubt 4th edition will depart from the "d20 higher is better" approach, but I feel 4th edition will be a radical departure from the current approach to character class construction. Perhaps rather than more classes, it will have fewer. Build your own character from four (three?) base classes: Rogue, Warrior, Mage, Priest (mage and priest could be "caster" instead). Add to this the combination of character development feats and skills that create the flavor you want.
Spellcasting is unlikely to change a great deal, but I would assume a major overhaul in metamagic. The current system is getting bloated. If we do see a big change in spells, it would likely be the introduction of spell points as core.
Feats in general may see a major overhaul or revision, bringing Bloodlines, Regionals, General, and Trees into a more unified approach. If the simplification / customization of character development occurs, the Feat system will be integrated with this.