This example is a bit corny, but you might like to think about the way in which films such as Independence Day present us with a very powerful notion of American nationalism and the American spirit. I agree that reading Imagined Communities would be a great place for you to start. It is a true classic and asks the big question of why it is that people are willing to lay down their lives for an illusory notion. You also might like to examine the way that films have been used as propaganda during war time. Another famous example is the Laurence Olivier film version of Henry V which was brought out during WWII and is very jingoistic.
One of the most famous films ever made is called Triumph of the Will, directed by Leni Riefenstahl. It was designed to celebrate Germany as a fatherland during the rule of Adolf Hitler. Many films produced during the Nazi era had the same purposes, but Riefenstahl's film is considered one of the most powerful pieces of political and nationalistic propagranda ever made. Many films produced in the Soviet Union had similar purposes (the great composer Shostakovich was pressed into service to write music for some of them). Nearly all totalitarian regimes produce films of this kind and may therefore be worth finding. You may also want to have a look at a film called The 49th Parallel, made during World War II and set in Canada. The music was done by the great British composer Ralph Vaughan Williams. If you watch the opening credits (available on YouTube), you will see a perfect example of nationalism in film.
That is a really broad question. For its theoretical part, you could refer to Benedict Anderson's important work Imagined Communities which defined nations as narratively constructed entities rather than real communities. The nation is more a mental image because its members cannot have meaningful face-to-face intereractions between them. It would be interesting to see how films contribute to build a common sense of nationality. As you also tagged your question with "India", you could also explore the concept of the nation in the works of postcolonial authors such as Homi K Bhabha, Robert Young (White Mythologies) and G. Spivak.
As for filmic examples, you could focus on how expatriates imagine their homelands from another nations and how the actual encounters with them affects their lives. See The Namesake (2006) directed by Mira Nair and based on Jhumpa Lahiri's novel (first link below is to the enotes novel study guide). You can find more examples of immigrants films following the second link. Another line of enquiry, could be how different nations have imagined their origins and foundation. The most famous and controversial example for this could be D. W. Griffith's Birth of a Nation (1915) which focuses on the impact of the Civil War on the United States (taking a somewhat pro-southern stance). Another example could be Mario Martone's We Believed (2010) about the process of Italian unification (Risorgimento) in the second half of the nineteenth-century. This is a particularly interesting case as it was released just as Italy was getting ready to celebrate its 150th anniversary as a united nation. Yet, the film was described by many as revising the national myth of the Risorgimento (you could perhaps contrast it with Luchino Visconti's Senso, 1954)
Could you please tell a bit more on this topic ? We need to make a presentation out of this..
Thank you so much!