There are a number of possible answers here. I suggest that you check your textbook to see what answer you are supposed to give. That said, I would argue that the second main thing that Mexico needed to do after independence was to set up a stable government. Sadly, it failed to do this and faced decades of conflict after independence.
Upon becoming independence, Mexico needed to have a governmental system that would unite the country and allow it to move forward in a unified way. However, it did not get this. First, it got a self-proclaimed emperor. After that, it got conflict between liberals who wanted more democracy and conservatives who essentially wanted rule by strongmen. This is one major reason why Mexico was unable to flourish in the years after independence.