It seems a bit unusual to address themes in an ironically comic satire along with the themes in one of literture’s greatest tragedies, but, ironically, there is some overlap of themes. In Ben Jonson's ironic comic satire, The Alchemist, the major themes that may overlap with Shakespeare's tragedy, Julius Caesar, are those of change and transformation; greed; deception; and victim and victimization.
The major theme in Julius Caesar is critically recognized as being that of the authority of the ruled to perpetrate regicide to remove a tyrannical (or potentially tyrannical) ruler. Critics state that this was a theme that was an issue in Elizabethan England just as it had been an issue in Brutus’s Rome.
Queen Elizabeth I had undergone two attacks to her rule, one from the Earl of Essex in 1601 and another later from radical Puritans like Peter Wentworth and John Field who sent up a cry for democracy instead of monarchy. They called for "liberty, freedom and enfranchisement." These are words that Shakespeare gives to Cassius to say while speaking with Brutus in Act III, scene i:
Some to the common pulpits and cry out
“Liberty, freedom, and enfranchisement!”
People, and senators, be not affrighted,(90)
Fly not, stand still; ambition's debt is paid.
Within this major theme are several thematic sub-components that overlap with various themes in Jonson's The Alchemist. The most obvious example is that of victim and victimization. Jonson looks at the theme from a social perspective through the premise of victimizing con-men conning their way to wealth over the wreckage of those they victimize (innocently enough, right?, by offering magical potions). Shakespeare portrays the ultimate victim, the ruler of Rome, felled by the worst of victimizers, his loyal and trusted friends and comrades.
Another example of a theme that overlaps is that of greed. The lovable (?) rogue swindlers in Jonson's play are greedy for wealth and devise a very clever way of using the master's house, while he is escaping the city by retiring to the country, to lure in innocents to swindle out of large sums of money--the larger the better--by pretending to offer alchemical remedies for what ails them, which often also revolves around their own greed:
SUBTLE, THE ALCHEMIST: Who is it, Dol?
DOL: A fine young quodling.
My lawyer's clerk, I lighted on last night,
In Holborn, at the Dagger. He would have
(I told you of him) a familiar,
To rifle with at horses, and win cups.
In Shakespeare's, it is greed that would compel Caesar to accept a king's crown; it is greed that motivates Cassius; and it is his greedy motives that prove the downfall of Brutus as well.
The major themes of Shakespeare's Julius Caesar is the question as to whether it is ever defensible to kill a king. The Elizabethan audience for which Shakespeare wrote this play generally believed that killing a king was a mortal sin: something we see in Shakespeare's Macbeth (where it is not justifiable) and Hamlet (where a son has promised to avenge his father's death by killing the present king). In Julius Caesar, Brutus joins Cassius and his fellow-conspirators to kill Caesar; Cassius is motivated by jealousy, whereas Brutus is motivated completely by what is ultimately best for Rome; while Brutus loves Caesar, he loves Rome more, and fears that Caesar will destroy Rome. The battle of philosophies present during Shakespeare's time contemplate the sin of killing a king, against the need to kill a king to save the occupants of the kingdom from enslavement "to the will of a single man"—is this kind of murder justified when "liberty is at stake?"
Ambition is another theme of the play. It is not seen in the person of Brutus who acts simply for the good of Rome and not for himself, but it is present in Caesar and Mark Antony. In casting doubt on the actions of Brutus, et al, Mark Antony moves the sympathies of the crowd away from Brutus to support Antony instead.
On the other hand, the themes in The Alchemist are very different. They are magical realism, numerology, patience, perception, religion/spirituality, transcendentalism, and water.
Whereas Julius Caesar is one of Shakespeare's historical plays dealing primarily with political strife in Rome (with very little of the supernatural), The Alchemist is rife with it. Julius Caesar concentrates on how one man or many can decide the fate of a nation, whereas Santiago in The Alchemist is concerned with achieving his personal legend and finding happiness. The supernatural is significant in how nature and man are one.