We all know the penis develops during puberty in response to certain androgens (HCG, HGH, testosterone amongst others), but what determines when the growth stops? A fully developed male still has considerable levels of these androgens in his blood, yet the penis has stopped responding to them with tissue growth. Females respond to high levels of these hormones with developing clitoromegaly (their clitoris grows in size) and the clitoris is the female homologue to the penis. Likewise, boys and men with micropenis also respond to the aforementioned androgens with growth of their penis. There is some sort of mechanism that stops the cells of the penis from responding to these androgens once a man has reached maturity.

If this mechanism could be reversed or blocked, we could achieve chemical penis enlargement by way of hormones. What makes me think this could be realistically achieved is the fact that scientists have reversed the sertoli cells of rat testicles (sertoli cells are supportive cells that proliferate before puberty and determine the testicle size in the adult individual) back to a proliferative state. The sertoli cells started proliferating in the adult rat testes which grew the testicles. It doesn't seem too unrealistic that the cells of the penis could also be reverted to a "young" state where they respond to stimuli like cells of a pubertal boy would.

High quality penis enlargement is unironically on the horizon, I'd guess within 10 years.