Whether it is making unmade prototype accessories, or mashing up two of your favorite toy franchises.  Toy customizing is fun and there are many ways to achieve awesome results.  Kit-bashing (mixing and matching parts from different action figures is a great method used by many.  Since the pandemic I have gotten into this as well as now digital kit-bashing.  This is a non destructive alternative if you have access to a 3d printer.  I'm not going to tell you to not chop your toys up like Sid in Toy Story, I really enjoy tinkering with toys and seeing the creative ways others come up with unique figures.  

A lot of people also make use of "Apoxie Sculpt" or "milliput" putty as a way to sort of sculpt on your own details on top of existing figures.

Using articulated 3d scans of figures is great for doing digital kitbash work or making accessories that work with existing vintage figures.

The two un-produced accessories to the left were made by my friend @epoxy_crusader and I using a free cad software called "Tinkercad"  This program does not require a download and can be used to model accessories or hard surface designs quite well.  You can also import ".SVG vector files" into it and bring logos or symbols from 2D into 3d and export .STL (3dmodels) to be 3d printed!

Collection of some of my favorite TMNT Customs and Customizers

My Customs and Kitbashes

Resources for getting started yourself

Digital Kitbashing