In real life, Nami's dream is the most basic and easy to achieve.
All she has to do is travel around the world and record the map of every location. Like a tourist.
As for the one that is very silly in modern days society, probably Luffy's or Sanji's. Simply because the One Piece/All Blue doesn't exist in real life.
Chopper's dream of curing every diseases is also very difficult to achieve but with improving technology and access to better health care, I think Chopper will eventually cure every disease including cancer. Just that it's gonna take a while.
Dragon's and Crocodile's goals are stupid but it's practical in real life.