One interesting element is the combination of different power systems. In 'RWBY', there are the unique Semblances and Dust-based powers, while 'Mass Effect' has biotics and advanced technology. When combined, it can lead to really cool battle scenes and power-ups for the characters.
Another concept is that the Reapers from Mass Effect are interested in the animatronics. They see the ability of the animatronics to keep 'coming back to life' in a sense as a useful trait. So they start to experiment on them, which causes all sorts of chaos as the animatronics' programming and the Reapers' technology clash.
Character crossovers are also a big theme. Writers like to see how characters from RWBY would react to the situations and characters in the Mass Effect universe and vice versa. For example, Ruby meeting Shepard could lead to some really interesting storylines.
One interesting Mass Effect 3 fanfiction could be about the alternate endings. For example, a story where Shepard manages to find a completely new way to end the Reaper threat that wasn't in the original game. Another could focus on the relationships between the different species in the Mass Effect universe post - Mass Effect 3 events. Maybe a story about how the krogan and the salarians are rebuilding their trust after the genophage cure.
For me, the pairing of Miranda and Emerald could be really exciting. Miranda's genetically - enhanced abilities and her strategic mind could be a great influence on Emerald. Meanwhile, Emerald could show Miranda some of the more underhanded tactics used in RWBY's world. Also, Kaidan and Ren would be an interesting combination. Kaidan's biotic powers and Ren's ability to control his Aura could lead to some interesting experiments in combining their powers. They could also bond over their more reserved personalities.
One popular site to find fanfiction for both is Archive of Our Own (AO3). You can search for 'Mass Effect RWBY crossover' on it and explore the available works. There are often interesting stories that blend the universes in unique ways.
The evolution of AI is also a common concept. Fanfiction writers may imagine how Mass Effect AIs could evolve over time, perhaps developing new forms of intelligence or abilities that are far beyond what was seen in the game. This could lead to all kinds of exciting storylines, such as AIs becoming the saviors or destroyers of the galaxy.
Some 'oc mass effect fanfiction' focuses on the personal growth of the OC. For instance, an OC starting as a low - level mercenary in the Mass Effect universe. Through a series of events, they learn about the true nature of the galaxy, form unexpected alliances, and eventually become a key figure in the fight against the various threats, evolving both in their combat skills and in their understanding of different species and cultures.
Well, some might find the exploration of character relationships in a more intimate way interesting. It can show different sides of the beloved Mass Effect characters that the original game might not fully explore. However, it's important to note that such fanfiction often strays from the main narrative focus of the game, which is more about the grand adventure in space, combat, and inter - species politics.
One interesting crossover could be with a fantasy RPG world. The Mass Effect characters might encounter magic and different races from the RPG, leading to unique storylines. For example, Shepard could learn magic spells and use them in combat alongside biotics.
One interesting storyline could be about a character forming a unique bond with different species in the Mass Effect universe within the harem setting. For example, they might start with a relationship with an asari, exploring their long lifespan and biotic abilities. Then add a turian into the mix, dealing with the cultural differences and military discipline. Another could be a story where the protagonist has to navigate the political intrigue of the various alien races while maintaining their harem relationships.