Homunculus Jar...

This is an idea I came up with for a P&P role-playing game some years ago. I've used it in several games subsequently and it always provides players with some dark delight.

The idea basically works like this. A lot of games provide mages with the opportunity to summon a familiar (MMOs in particular have developed this idea further, and many classes have the ability to control 'pets').

The Homunculus Jar basically provides mages (in my games, but could be extended to be adopted by other character-classes) with a series of sideline missions or sub-agendas during which they attempt to find useful components for their Jar. Useful components are basically body parts of creatures. These are placed within the jar, and dependant upon the mages power and success in this, the mage can effectively create a 'frankenstein familiar' - magically stitching together components from other beasts to create a unique and multi-faceted entity.

I had one player who took bat wings, venomous fangs, toxic spines, multiple limbs and a goblin foetus to construct a tiny winged, six-limbed goblinoid which poisoned its victims with its fangs and spines.

There have been many and varied examples.

Obviously the freedom of P&P supports this concept wholeheartedly. But a stripped down version could be employed by MMOs.