I first saw this idea about putting Reagan on the Trillion dollar coin proposed right here today, and while this idea was likely proposed in jest, it is actually a quite good and quite feasible idea, and I really mean it!
I have been a coin collector/investor for over 30 years, and here is what I know about the process of putting images of American presidents on US coins.
Originally, America used abstract conceptual imagery on its coins, starting with many different abstract artistic depictions of the concept of "Lady Liberty".
Then, America started depicting Native Americans (so-called "Indian head" coins) on many of its coins. The history & sick irony of America depicting Native Americans, (a people whom America committed a genocide against) on American coinage is a subject which deserves its own diary, but for now, consider the humble "Buffalo nickel" US coin, introduced in 1913. Its front side (technically called the "obverse" side), was a head bust of a Native American, and its back side (technically called the "reverse" side), was a depiction of a Buffalo animal. Great. How ironic. America ruthlessly extinguishes both the Native American and the Native American's prime food source, the noble Buffalo, and then pays honor to both via the same American coin!
(read more below the orange ploopie.)
In any event, the first US coin to bear the image of an actual US president was the Lincoln head penny, introduced in 1909. From that point on, images of US presidents have gradually emerged to be the dominant (but not exclusive) paradigm for US coinage:
In 1932, George Washington first appeared on the US quarter;
In 1938, Thomas Jefferson first appeared on the US nickel (replacing the Buffalo nickel);
In 1946, FDR first appeared on the US dime, soon after his death in 1945 (Note: FDR was the first modern US president to be depicted on US coinage, he was that great);
In 1964, JFK first appeared on the US half dollar, soon after his death in 1963.
By law, a US president must be deceased to be honored on US coinage, so you cannot put, let's say. GW Bush's image on a US coin, but since the repubs consider Reagan, who is deceased, to be right up there greatness-wise with Washington, Lincoln, Jefferson, FDR, JFK, and so-on, then why not put Reagan on the face of the Trillion dollar coin?
Doing such could also be very savvy politically, in that doing such could perhaps make the Congressional repubs more accepting & supportive of the Trillion dollar coin; after all, how could the repubs/Fox News mock/reject a US coin proposed to honor & bear the likeness of their great Ronald Reagan?
Such a coin would be called the Ronald Reagan Trillion Dollar coin.