What new things introduced in 4th Edition should be around in 5th?
- At-Will powers for everybody (even if they're merely Move actions such as the Thief's "tricks").
- FORT, REF, and WILL as defenses, not saves.
- Power Sources formalized, allowing such phrases as "any arcane class" to be a Prerequisite for varieties of stuff (feats, paragon paths, epic destinies, etc.). (This still needs further development.*)
- Backgrounds. (PHB2)
- Hybrid Multiclassing. (PHB3)
- Skill Powers. (PHB3)
- Themes. (ongoing . . .)
*Re: further development of power sources: The simple way to reduce the clutter of powers is to put them into combined lists, one for each power source. Here's an example of what I mean:
Bard -- "Vicious Mockery": Charm, R-10, single target, CHAvWIL; 1d6 + CHA psychic, and takes -2 tohit UEOYNT.
Wizard -- "Illusory Ambush": Illusion, R-10, single target, INTvWIL; 1d6 + INT psychic, and takes -2 tohit UEOYNT.
Here, the only differences are (1) Charm vs. Illusion keyword, and (2) CHA vs. INT modifier for tohit and damage.
WotC could introduce a new keyword, such as "Primestat," to generalize powers, where the Primestat of a Bard would be CHA and the Primestat of a Wizard would be INT; then replace the specific wording in the class-specific versions of the power with the "Primestat" variable. (So: "Attack: primestat vs. Will.")
Also, WotC could introduce a new keyword, such as "Influence," to generalize power types, where the Influence of a Bard would be Charm and the Influence of a Wizard would be -- whatever that particular Wizard chose to take as his or her Influence.
That kind of thing would allow for a bit of consolidation of power lists by power source; but would still leave open the 3E-style option of having each listed power state which classes can take them.