Take a look at the PNG spec. This format has some very good rationale behind it.
Also, decide what’s important for your future format: compactness, compatibility, allowing to embed other formats (different compression algorithms) inside it. Another interesting example would be the Google’s protocol buffers, where size of the transferred data is the king.
As for endianness, I’d suggest you to pick one option and stick with it, not allowing different byte orders. Otherwise, reading and writing libraries will only get more complex and slower.