*There are a ton of classes/prestige classes from 3.x that I'd love to see as new classes/subclasses.
*4E saving throw where Fort, Ref and Will are tied to two stats and you pick the better of the two.
*Those 3.5 multiclass feats that basically allowed you count a specific class as another class for the purposes of progressing a class feature; like Ascetic Hunter - Monk and Ranger levels stack for unarmed damage.
*ASI/Feats bound to character level instead of Class level.
*Being able to upgrade to a better animal companion/familiar.
*When creating a character, a positive Intelligence modifier granting extra languages and skill points. I'd be happy if for each point I had, I got to pick between a Knowledge skill, Language, or non-weapon/armor proficiency.
*More uses for gold. The Magic Item Compendium [3.5E] and the Arms and Equipment Guide [3E]. (hey can keep the items that would mess with bound accuracy.
*MAGIC ITEM SHOPS. They can keep the attunement rules. They can have it be stocked with Common and Uncommon items with a small chance of Rare or better.
*Weapons of Legacy [3.5E]
*Settings other than Faerun