From BlenderWiki

Bullet Integration TODOs:

This is a list of things I'd like to have have for the rigid body simulation.
I'll work on all of them eventually but would be happy to get patches from others of course :)

General

• Add option to calculate mass automaticall (based on volume)
• Improve integration with the animation system
  • Remove/Reduce cases thac cause 1 frame of lag.

Rigid Body World

• Allow world to be scaled
• Look into using bullet's multithreading capabilities

Rigid Bodies

• Add proper activation system (instead of using rigid body deactivation)
  • Add activation conditions (On collision, on time, ...)
  • Add ghost objects that can be used as geometry triggers
• Look into using ccd
• Add effector weights for individual rigid bodies

Force fields

• Account for timestep (right now forces get weaker the more steps per second are made)
• Implement follow transformation force field (moves, rotates objects when moved, rotated)

Collision shapes

• Object origin should be center of gravity for all shapes (may not be possible)
• Add convex decomposition shape
• Add compound shape
• Add non uniform sphere shape (maybe)
• Add plane shape (maybe)
• Add internal edge utility for static bvh mesh shapes
• Take shape keys into account when creating shapes
• Add option to update mesh shapes while the simulation is running
• Convex hulls should have simplification options (like max verts)
• Add collision shape visualization

Constraints

• Add hinge 2 constraint
• Add cone twist constraint
• Add universal constraint
• Add spring constraint
• Add motors
• Add constraint visualization
• Add utilities to connect selected objects with constraints

Point cache

• Allow point cache rna functions to work with scenes (needed for some ui properties)
• Add more depsgraph dependencies so cache invalidation works when combining rigid bodies with other simulations