I mean, I sat on that keychain for close to 5 years before doing something with it, so inspiration can come back randomly from a bits box!
I'm interested to see where you take your ideas!
Extending limbs should be pretty simple, all you need is a decent pin vise, a box of paper clips and some epoxy putty/greenstuff to sculpt new limbs. I personally used Alteco Epoxy Putty for DIY repairs since I like its strength. If you do buy one, make sure the Resin and Hardener tubes are separate and not mixed together, especially some of the plumber's putty which tends to heat up in a chemical reaction when mixed together.
I'd also advise in investing in some cheap
silicone shapers. The one I linked is a cheap set of 5 I bought off Aliexpress and they've served me well in basic sculpting for two part putties, especially once you add some vaseline to get rid of the tackiness (I would especially suggest these getting both shapers and vaseline if you want to blend the putty smoothly onto the plastic)
If you need a good guide to work with epoxy putties/green stuff when sculpting for miniatures, Tom Mason has a phenomenal set of
videos that can guide you through the process from building an armature all the way to muscle definition on a small scale.
If you're looking for ball-joints, I can vouch for the ones from Hobby Base, I've been using them in various projects for a decade and they've never let me down. Here's a link from
Amazon since AmiAmi currently has its preorder list closed.
Hope that helps!