It does not matter what order things will happen but a few things set in stone are:
Luffy will surpass Shanks by getting a bigger crew and finding OP
Zoro will surpass Mihawk by beating him
Shanks is the benchmark (the freest currently, the man nearest to PK, the Roger of current gen) for Luffy
Mihawk is the benchmark (the WSS ever) for Zoro
Zoro can fight Mihawk before or after Gorosei, it is completely up to Oda how he wants the flow to go. Beating someone later does not mean that the previous one was weaker (couple of examples mentioned previously by others).