There are many topics that a marketing research paper can be written about. Here are just a few ideas relating to marketing promotion, place, products and price.
You could do research on the promotional use of social media in marketing today. It has been around long enough now for there to be data available and many articles written about this phenomenon. You could discuss its advantages and its disadvantages. Social media is only one promotional strategy.
You might explore the relationship between social responsibility and marketing. Make no mistake: social responsibility is related to promotion. Every time a company does something good in a community or for a cause, it is promoting its image and its product. To what degree this helps in marketing is something you could explore.
Pricing strategies are part of marketing, too. Whether something will sell or not sell is going to be based upon choosing among a variety of strategies, to undercut the competition, to price high to create some "snob" appeal, or pricing high for something so new that no one else has it yet. All of these pricing strategies can be researched.
How goods are to be distributed is a marketing decision related to place: how the product gets to the customer. They might be sold directly. They might be placed in big box retail stores. They might be distributed in small, company-owned boutique stores. This is an important marketing issue.
Packaging is an important aspect of marketing that relates indirectly to product. Customers are attracted to a particular "look," or want convenience and durability in their packaging. There are studies on this, and countless examples to discuss.
Advertising is central to promotion. Whether advertising should be local, national, or international is an important question. Even for a national company, tailoring promotional advertising to local or regional markets can be beneficial. Many national restaurant chains tailor their ads to local or regional markets.
International promotional marketing is an area ripe for research, and there are no doubt thousands of articles that explore this topic. For example, there are many case studies of marketing failures where a lack of knowledge of another culture or language created embarrassing advertisements or a product that simply could not be sold in that culture.
Companies may have different promotional marketing strategies for male and female markets. Look at the advertising all around you. Is it aimed at males or females in particular? To what degree does the gender of the market segment aimed at affect promotional marketing decisions? A company might use different distribution channels, relating to place, for the two different markets. These are areas that could be investigated.
These are just a few ideas. One might appeal to you or lead you to think of an idea you like in the field of marketing.