Stuff I'd like to see in 5E

- Altered Skills: the 3.5 Skill System felt like a point-based System like Gurps attached to the core mechanic. That's not a bad thing, but they should extend the use of the skills. Make them useful in combat, give extraordinary advantages. Maybe even incorporate the combat system into the skill system. Why can I hit harder/more accurate automatically by leveling up, but my skills have to be bought by points?
They changed that with 4E, but the skill system in 4E always felt lacking to me. It didn't allow for diversification and in my opinion the skill list itself is lacking.

- Multiclassing: This is a tricky one. On the one Hand I like 3.X multiclassing, but on the other Hand it it a mess. You couldn't properly multiclass as a Caster without nerfing yourself into oblivion, but on the other Hand these Frontloaded Classes like a Paladin had no intention not to multiclass. Prestige Classes were a good idea initially, but it led to Prestige Class Hopping where people were just collecting class features.
Legend's multiclassing is a nice idea, similar to what I had homebrew some time ago. Maybe expand on Alternative Class features and go from there?

- Saving throws: I really like the defense mechanics from 4E. I think they were a good idea

- Classes: Please no generic classes! The fighter is in my opinion not a class, but a role in combat. It is the guy that swings a sword, but nothing more. On the other hand we got the Paladin, the Ranger or even the Rogue, where we have a clue what they do out of combat. Their class defines their combat style, not the other way round.
Again, maybe something like the AD&D2E Kits: Define a base mechanic (warrior, wizard, priest, thief, psionicist, etc) and build classes on top of that. There should be a mechanical difference between these classes. They don't need to be absolutely balanced against each other. Let them handle it differently, or maybe let them even be incapable to do something specific.

- Optional Rules: Make the System modular! For easy play use the basic rules, but for a more realistic/gritty/complex/balanced play use optional rules.

- Bring back vancian casting: Many, many players dislike that one. But let's face it: It is easy to understand and easy to play. It adds complexity that some (many? At least I do!) enjoy. It screams DnD to me like nothing else.