At the end of Wuthering Heights, what was the relationship between Cathy and Hareton and what were their future plans?

Cathy and Hareton have developed a strong, loving relationship, and will undoubtedly soon be married. 

Although Cathy has long scorned Hareton, she finally relents, making overtures of friendship and offering to teach him to read.  Hareton, indignant that she blames their past enmity on him, at first rebuffs her, but he soon softens, and the cousins establish an amiable truce which quickly develops into something deeper.  The two overcome their biggest obstacle  when Hareton forbids Cathy to speak ill of Heathcliff, and Cathy, with a growing maturity based on love, decides it would only be cruel to persist in trying to make Hareton see that Heathcliff has treated him abominably, realizing that "he was attached by ties stronger than reason could break". 

As she watches Hareton and Cathy huddle like innocent, happy children over a book in Chapter 32, Nelly relates that their "intimacy thus commenced grew rapidly...both minds tending to the same point - one loving and desiring to esteem, and the other loving and desiring to be esteemed - they contrived in the end to reach it".  She sees their eventual union as ineveitable, calling it will be "the crown of all my wishes".  In Chapter 33, Heathcliff also recognizes the obvious conclusion, and reflects on the ultimate irony of how his lifelong quest to destroy the Earnshaw and Linton families has ended.