What are some good Spanish idioms to include in my research essay?

Although it depends what your research essay is on (i.e. you can't use the same idioms if you are writing an essay on a novel character and on a historical event), there are a few good ones that can be used to show off your knowledge of Spanish in the majority of essays:

1. Ver las orejas al lobo - To see the danger signs (Really good also for cultural / contemporary ones in the discursive essay, or when discussing a character's relationship with another character.)

2. Al fin y al cabo - Finally / At the end of the day / When all is said and done (Good for conclusions.)

3. Valer la pena - To be worth it / To be worthwhile

4. Tener en mente - To keep in mind (Good for comparisons, or when transitioning from advantages to disadvantages. E.g. "Sin embargo, necesitamos tener en mente que...")

5. Ir de mal en peor - To go from bad to worse (You can use this when talking about a character's progression, or even about a negative event in history. It's rather versatile.)

These idiomatic phrases are particularly good as they are not that informal, and at least one can be used in every single essay!