How do I make a filterable Fields Fieldtype like FieldtypePage?

[hellomoto]

I want to create a fieldtype Fields which in the config can be set as single or multi-value, and optionally filter available options dynamically according to a specified template field (using something like FieldtypeTemplates or FieldtypeSelectExtOption or even FieldtypeText). FieldtypePage can do this by including another page field in the selector, it can be a child dependent on its value... Could anyone just provide me with a starter guide? I'm looking at other modules for insight but ultimately remain lost. Thanks so much.