For playing the time , like Super Mario on SNES, it can take lot of time, like for Metroid or Zelda on Snes !
Or Wonderboy on Genesis !
A cool thing could be map with plot levels (like mario on Snes) where you can choose where to go , mixed up with power ups !
And you could have permanent power ups and temporray ones like in Mario.
Some parts of levels could only be discovered/explored when
you have the appropriate level up !
In terms of gameplay : progressive level up, new weapons/powers
is what can make you playing the game to the end !
Juts one example of very good 2D design and awesome gameplay : and it's not some engine as powerfull as A7 behind
FinWick game 
