Answer by Debbie Boykin:
Caffeine is a phosphodiesterase (PDE) inhibitor and nonselective adenosine antagonist. Many of the nootropics mentioned in this article () will have a synergistic effect with both racetams and caffeine.
Specifically, I think l-theanine, theobromine, and theophylline–all naturally abundant in tea- also inhibit PDE and would likely augment caffeine. Moreover, any of the racetams will boost excitatory glutamatergic signaling and synergize with caffeine. Some neuropsychiatrists have speculated that nicotine is the only substance that reliably enhances memory in almost all subjects. Nicotine is commonly used alongside caffeine, e.g., many smokers are heavy coffee drinkers as well.
TLDR: tea xanthines, nicotine, racetams, certain amino acids like glycine, d-serine and tyrosine, and modafinil would likely augment the procognitive effects of caffeine.