Thanks guys ! =D
I added a bit of bloom and tested something for dynamic shadow casting:
The problem here is that since I'm using only prerendered 2.5D assets, I do not have access to the full information about the geometry of the objects. It was not too much of a problem for the directionnal shadow casting since I assume the sunlight to be oriented with a direction close from the view. But for dynamic lights that move in the scene, it is clearly not enough (and also, it need to be recomputed quickly, which is not possible with the way I'm doing it).
Hence, I decided to add some invisible "3D objects" to the scene, that act only as shadow casters. The idea would be in the game to add only those for big, simple geometric entities (like walls, pillars, barrels, etc).