I'm not even sure NWPs fit as distinctly 2e, as nearly-identical mechanisms showed up in late 1E (OA, WSG, DSG).
Great question! The whole push for backwards compatibility means there's very little under the sun that is truly and uniquely 2E AD&D, at least until the Player's Option series. There are a few rebuilt systems where the specific implementation isn't found in 1E(/OSRIC) or in B-BECMI (and thus shows up in OSR material). Off the top of my head (and here just identifying them, not weighing in on whether I'd want to see them), I thin there's:
- AD&D 2E's initiative rules
- unarmed attack, grappling, and martial arts re-works (a few attempts, IIRC)
- Priest Spell spheres
- Specialty Priests
- Bard and Thief player choosing of ability % distribution
- Kits
- The Psionicist Class
- Wild Magic
- 'historic' wordlbooks (the green-bound leatheret splatbooks)
- PO: Spells and Magic alternate spellcasting rules
- Some PO: Combat and Tactics combat rules (ex. attacks of opportunity)
Most all the rest are seen elsewhere. As I mentioned, NWPs are late 1E. As are specific campaign worlds with their own rule changes*. Playing as non-human/non-demihumans shows up elsewhere**. Non-illusionist school specialists (evokers, etc.) are new to 2e, but with the precedence set by illusionists (and Dragonlance robe-based specialties), I'm not sure if they can be called new or not. Psionics have been around since supplement-era oD&D.
*BECMI also has with Hollow World
**although the Complete Humanoids Handbook might have been the first that allowed high-powered creatures like Ogre Magi to be played at 2-3x xp cost, that might be new
Personally:
- Of these, I would love to revisit Specialty Priests, Kits, Wild Magic, and Psionicists.
- I think specific OSR game or setting books would be great places to explore a Viking- or Charlamagne setting, or use an alternate magic style.
- I think things like thieves/bards distributing their %s should be part of an overall re-think on their abilities (perhaps in conjunction with a reassessment of the skill systems in general). I think a lot of OSR games do play around with this, specifically because it's part of the TSR era on which a lot of people have mixed feelings.
- Likewise, certain rules like grappling and zone of control* are those that the game never really got right, and I think a lot of people are already working on those.*i.e. 'why not rush past the fighters to kill the magic users once play leaves 10' wide corridors and parties with 6-600 hirelings and retainers?'