I think that there should be more missions you can get from you sensei, and that they should be longer. Heres an idea:
You ask your sensei about special missions
Your sensei: We just got an A rank mission to protect a actress starring in a movie.
Will you take this mission
You answer yes
You meet the actress, and walk to the ship to the land of snow (sound familair)
On the ship you are allowed to buy some weapons and heal yourself
You leave the ship and walk with the actress
You go into a tunnel and you are forced to fight a train
Train level: (One above you)
Attack: 0
Hp: 50 more than you
defense: 3 more than you
You have a set amount of turns to beat the train or it will kill you
You beat the train, and learn that the actress is actually a princess!
Suddenly a snow nin comes and attacks you
Snow nin: 2 levels above you
HP:25 more than you
Attack: 3 more than you
Defense: 3 more than you
Finnally you beat him. You take the princess to the dropoff point and she pays you some huge amount of gold and gems
Well anyways thats my idea. It may be too long and it can be edited however
Thanks for reading