I haven't seen anyone mention it yet, but: 5E psionics. Psionics have a long history in D&D, and a lot of fans as well as detractors. It's likely just a matter of time until we see them in 5E, but the question is when and how.
My guesses:
Speculate away!
My guesses:
- It won't be given a full treatment in either the Player's Handbook or Dungeon Master's Guide. 5E is supposed to avoid splatbloat, but even then psionics is probably best suited to being an optional subsystem presented in its own book;
- By December 2015, we'll either have psionics rules or know when they'll be published;
- It'll appear prior to or alongside the launch of a setting like 5E Eberron or 5E Dark Sun (because those are two settings that include psionics more heavily than most);
- The psion class will still use power points;
- "Psychic warrior" and "soulknife" will be new subclasses for existing classes (fighter and rogue, probably);
Speculate away!