Custom Model format and Game Engine

[Demonsangel]

Say you would want to create your own model format file/container, what would be required to achieve optimal performance?
I've had a look at a few formats over the past few months and I've seen quite some differences: Some store the armature in/out the file, the way they look up the triangles, ...
but I was wondering if there is a decent read somewhere about creating your own format, the requirements and tips and the same for creating the actual engine.

I've read some research, but it mostly what I already know (what's inside a 3d model, ...) or about features that aren't a necessity right now.