The theme that I would want to talk about when analysing these two texts would be that of fate. Clearly this is highly significant in both texts, though this is used very differently in either story. In The Alchemist, for example, fate is used to in positive ways to depict divine powers as being benelovent and wanting the growth and development of people such as Santiago. In fact, Santiago's increasing maturity is deliberately linked to his ability to tune into the Language of the World as he is able to be more receptive to bigger and more important visions and omens that show fate's hand in indicating future areas of danger.
In Romeo and Juliet, however, fate is depicted as a force that is implacably opposed to the union of Romeo and Juliet. This is something that is indicated at the very beginning of the play in the Chorus's opening lines:
From forth the fatal loins of these two foes
A pair of star-crossed lovers take their life,
Whose misadventured piteous overthrows
Doth with their death bury their parents’ strife...
References to "star-crossed lovers" and, in later scenes, quotes such as being "fortune's fool" and "defying" stars show the way that Romeo Juliet try--and fail--to valiantly oppose fate and make their romance work. Fate, however, unlike the benevolent force that it is shown to be in The Alchemist, in this play is a force that dictates the certain fixed futures of characters that cannot be gainsaid.