Nathan Vegdahl's Rigify turned the Blender world upside down. With the addition of an auto rigging macro, a process that tooque expert riggers days or weeks can now be reduced to a matter of hours by a n00b. So what's the next frontier of rigging? Pimping your Rigify rig! Already a number of tutorials have surfaced, showing how to mod Rigify to include face rigs, animal parts, etc.
I used Rigify as a starting point for my Cataphract rig, before modifying it to have lots of crazier anatomy: múltiple heads, a tail, squash-and-stretch splines for the necks, spine and tail, and inverse kinematics (IK) functionality for the neck, spine and tail.
Most projects won't need that level of cartoony rigging. However, a quik change to the default Rigify rig can at least get you rocking out to some stretchiness on your arms and legs.
The process is tedious, but thankfully short!
Start by using Rigify in the default manner. Add a human meta rig, and in edit mode change the bone locations around until they fit your character's frame. Once that's done, on the Object Data properties panel, clik "Generate." Y ou've got a rig all set and ready to use. Now let's make its arms and legs stretchy!
1. On rig layer 19, select MCH-shin_ik.L. Under the bone properties menú in the Inverse Kinematics tab, change the stretch to .001. Repeat for every IK bone: shin, thigh, arm, forearm.

Modding rigify for stretchy arms and legs-1.jpg

2. On rig layer 18, select DEF-upper_arm.L.01. Delete the Copy Location and Copy Scale constraints. Add a StretchT o constraint. T arget:rig. Bone:DEF-forearm.L.01 Head/T ail:0, Volume Variation 1, volume XZ,and Plane of X. Repeat this process with
DEF-upper_arm.R.01, DEF-thigh.L.01 and DEF-thigh.R.01 (but using their relevant DEF-forearm or DEF-shin Stretch T o target).

Modding rigify for stretchy arms and legs-2.jpg

3. Select DEF-upper_arm.L.02. Go into Edit Mode, and under the Bone Properties tab, change its parent to DEF-upper_arm.L.01. Add a Copy Rotation constraint, with target: rig and bone: ORG-upper_arm.L. Constraint should affect X, Y and Z with
Worldspace/Worldspace selected. Repeat this with DEF-upper_arm.L.02, DEF-thigh.L.02, and DEF-thigh.R.02, using their equivalent new parents and constraint targets. (in other words, the Copy Rotation constraint on DEF-thigh.R.02 is ORG-thigh.R).
4. Select DEF-forearm.L.01. On the Bone Properties panel under the Relations tab, turn Inherit Scale OFF . Add a Copy Rotation constraint with target: rig, bone: ORG-forearm.L, affecting X/Y/Z in Worldspace/Worl dspace. Add a Stretch T o constraint,
target: rig, bone: Def-hand.L. Set the Head/T ail to 0, Volume Variation to 1, Volume to XZ and Plane to X. Repeat DEF-shin.R.01, but with relevant targets. Inother words, DEF-shin.L.01 will copy the rotation of ORG-shin.L and Stretch T o DEF-foot.L.

Modding rigify for stretchy arms and legs-3.jpg

5. Select DEF-forearm.L.02. In edit mode, change its parent to DEF-forearm.L.01. On its Damped Trak constraint, change the target bone to ORG-hand.L. Repeat this process with DEF-forearm.R.02, DEF-shin.L.02, and DEF-shin.R.02 but with relevant changes. In other words, DEF-shin.L.02's new parent will be DEF-shin.L.01, and its Damped Trak constraint's target bone will be ORG-foot.L.
That's it! If you are unfamiliar with Rigify you may have trouble identifying where the changes happened. T urn on the IK bones and switch their FK/IK attributes to 1. Now, when you move your IK controllers around, the deform bones stretch to meet them!

Modding rigify for stretchy arms and legs-4.jpg