I'd like to see a particle system.
Presets for fire, smoke, rain, fireworks, flocks of birds, etc.

Systems for physics and wheel rotations.
There are some amazing implimentations of 2d physics out there...
http://motionboutique.com/newton/

Metaball type merging of vector objects that are positioned next to each other.
http://labs.byhook.com/2011/09/26/vector-metaballs/

It would be cool if a particle system interacted with other graphics in a scene.

eg. Snow falling from sky is blown by wind and settles on roof tops and shoulders of characters.
When the characters move, the snow on their shoulders falls off.

eg. Leaves start the scene attached to branches of trees.
leaves start to fall and collect on ground.
Wind blows and leaves fly 'off screen'.

Wow, that would be really awesome indeed!

Also, a 2.5 ragdoll function which hooks into the physics engine would be nice.

e.g. a cartoon character is standing center stage with a falcon sitting on his arm.
The man starts a monologue on the art of falconry.
As he talks a small bird flies past and the falcon takes off and zooms after the little bird.
Unfortunately the falcon has neglected to release his hold on the mans wrist and the man is hoisted into the air, and dangles beneath the bird as the falcon flaps off screen.

Character is scaling a mountain when he grabs a branch which breaks and the man flip flops down the sheer side of the mount.

2D Ragdoll system for Unity 3D
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2d Physics Demo v2.0
[url=https://www.youtube.com/watch?v=jBe5JOhszzw]

Rag Doll Physics (Verlet based)
[url=https://www.youtube.com/watch?v=KModFRhIpec][/url]
With a system like this CA could have a 'virtual puppet' mode where the program simulates that old fashioned puppet on strings look.

Check out the little car model in this demo...

Chipmunk Joints and Constraints
[url=https://www.youtube.com/watch?v=ZgJJZTS0aMM][/url]

Box2D mountain-bike...
[url=https://www.youtube.com/watch?v=bBIXpu-D_Zo][/url]