I see that they use Shift Z for this in Toon Boom. Shift Z to zoom in – and Shift Z again to go back out. I guess that you would prefer it different? I mean, one key only and using hold/release.
So a logical key could be the + key. Holding it will make the current zoom level go up x4. When you release the + key, it will pop the zoom back to where we came from.
Would that work for you Bahadir?
… Just came to think of a similar (but opposite) feature: Hold the – key to zoom out (to reveal the whole canvas), and when – key is released, pop back into the zoom level we came from.