slacer, its true, 3D side scroller(or any other kind of game) would not limit use of great light effect. and its possible (and great) to use postprocessing shaders to make a uniqe look and feel.

but i dont agree with "side scrollers are a bit "retro" and limit the player in a forward/backward trap". even in a side scroller we can let the player to move all around our world as we saw in Batman:arkham asylum (scarecrow levels) or early nes versions of ZELDA.

for example we can add a top-view camera to game and active it every time player reachs at a multiway crossing to let the player choose his/her way and after that we switch to side-view camera again and follow player. isn't that great and easy/fun to play. also this can be base for some puzzles.