Sebastian Alvarado
We have a lot of genome-editing tools – like zinc finger nucleases, or CRISPR/Cas9 systems – that could theoretically allow you to epigenetically seek out and turn on genes that make your muscles physically large, make you strategically minded, incredibly fast, or increase your stamina. Of course, at this point, these types of things have been explored mostly in lab mice, but it's fun to speculate.
About Bruce Banner
Bruce Banner's transformation into the Hulk would be incredibly traumatic to his body, and maybe his green skin is the result of a whole-body bruise. If you want to get really creative, maybe his blood is full of some sort of green Hulk-oglobin, which can carry more oxygen to the muscles than hemoglobin and gives him his strength and stamina.