it in a pot of basil. After that, she is almost always with the basil. She waters it with her tears. The basil grows strong and luxuriant, thriving far more than any of her other plants. The brothers notice this, and, in suspicion, steal the plant and find Lorenzo’s corpse. Afraid of being found out, they flee. Isabella dies mourning for her lost pot of basil.

Some themes you might want to consider include the following:

Emotions over logic—You might want to think about the all-consuming nature of Isabella and Lorenzo’s love, how Isabella learns in a dream about Lorenzo’s fate, or about the rationale behind retrieving his corpse and planting it in a flower pot. None of these things are “logical,” but they serve as the "logic" of the story nevertheless.

Death as escape—While neither Lorenzo nor Isabella wish to die, their deaths serve as a kind of reunion. Lorenzo’s appearance in Isabella’s dream also suggests that the truth of their love somehow can transcend death.

Unity with nature—Lorenzo literally returns to nature, first in his forest grave, then in the pot of basil. The image of Isabella nourishing the plant with her tears suggests that the basil is a kind of living intercession between herself and her dead lover. The plant’s vigor is symbolic of the the strength of their bond.

Nature vs industry—The brothers, for whom ”many a weary hand did swelt / In torched mines and noisy factories,” represent evil in the poem; they function not only to deny Isabella what she desires, but also subvert and pervert the natural order by murdering Lorenzo. If Lorenzo and Isabella represent nature and emotion, the brothers represent industry and logic. 

Class consciousness—While Lorenzo and Isabella seem unconcerned about their differing socioeconomic status, the brothers are keenly aware of it. As capitalists, the brothers can be understood to represent a kind of economic perversion, one which enslaves men like Lorenzo and subverts emotional norms.

Some of the more prominent signs of aesthetic philosophy behind Romanticism apparent in John Keats' "Isabella; or, The Pot of Basil" are (1) the preference for individual experience over universal experience; (2) the preference for powerful emotion and feeling; (3) the preference for emotion over reason; (4) and the preference for symbolism and suggestion over clarity of text. The opening lines of Keats' poem show the poem's orientation to the individual over the general and universal:

Fair Isabel, poor simple Isabel!   
Lorenzo, a young palmer in Love’s eye!

Lines 3 through 8 begin the journey of overflowing powerful emotion, representing both the subject's emotion and the poet's emotion:

They could not, sure, beneath the same roof sleep   
But to each other dream, and nightly weep.

Stanza IV reveals the orientation of the poem toward emotionality over reason:

So spake they to their pillows; but, alas,   
Honeyless days and days did he let pass;

Lines 69 and 70 illustrate the reliance on symbol over direct clarity of expression:

So said, his erewhile timid lips grew bold,   
And poesied with hers in dewy rhyme:

Further examples of each are apparent throughout but these early ones set up the orientation of the poem.

One of the revolutionary accomplishments of Romanticism, for better or worse, was to overturn the ancient and enduring presupposition, restated by Elizabethan Philip Sidney, that poetry is a divinely inspired vehicle for imitating the ideals on the heavenly sphere for the instruction of humanity. By emphasizing the individual and individual experience and making the poet the source of poetic inspiration as opposed to a divine poetic inspiration, the Greek and Renaissance philosophy of poetic aesthetic philosophy was reversed. Coupled with this was the idea of the preeminence of the poet, so the voice of the speaker of the poem came to be oftentimes the voice of the poet.

[For more infomation, see "Romanticism" by L. Melani, Brooklyn College, NY from which this answer is drawn.]