Btw i am also considering for my own Mod Hardmode Expansion.
But the biggest issue so far has come is which licence to pic.
To my degree, i would probably just put everything under a creative common licence, since this most likely represent the spirit of the community of stealing stuff from other modders like you see fit. (It would actually support such behaviour).
Another things is, i am not sure to which degree github can keep different versions of files. That would be most interesting especially for *.RMP, *.MAP, and MCD Tilesets. Does anyone know if there is a list of supported formats out there?
EDIT: Thanks for the Step by step guide, gonna take some time later to go throu it