Word has a billion features you did not know that exist. Getting something Word shaped is probably straight-forward enough (how long did it take to make Google Docs), but getting those dangling features and quirks would be a long haul.

Mimicking Excel - woof. This one is used by so many people in different ways, that unless you offer 1:1 bug compatibility, it would be challenging to get 100% of people to meet everyone's current use case.