So i think for each "Street" or "World" the game should tell you the recommend you should be when your on/at the street or world.
For example. You get to krokotopia and your level 11. (Btw this is a guess of the number) and then a message will pop up saying "Dear Wizard, we recommend that you should be level 15 when you get here" the you can click close and either go on or go back and get to the recommend level.
This message will only show up once at each place you need it. Even if you go there again it will not show up.
_________________ Other Suggested Idea _________________
Recommend level on each mission. So when you check your missions right at the bottom of the mission it will tell you the recommend level for this quest/mission so that way people will realize there level 10 and they should A. Get help to beat this. or B. Go back and level up more