What's the worst thing that can happen from my bruised toenail?

It can be: painful, the toenail could fall off, you could ruin the matrix and the nail may not grow normal, you may develop a thick toenail or fungus..

You asked.: Possibly lose your toenail, only to have it grow back with a permanent deformity from damaging the nail root. This could cause a painful nail condition that could lead to corrective nail surgery. While recovering from the surgery, the toe becomes infected and spreads to the bone. The infection gets so bad that the toe now needs to be amputated. All this from a simple nail bruise. Go figure.