The unique character design of Mysterious Heroine X Alter is a big draw. Her edgy and mysterious nature gives fanfiction writers a lot to work with. They can explore her backstory, her relationships with other characters in the Fate universe in really interesting ways.
Some Tumblr blogs are dedicated to Fate fanfictions as well. You can use Tumblr's search function to look for Mysterious Heroine X Alter fanfictions. But keep in mind that the content on Tumblr can be a bit unorganized compared to the more dedicated fanfiction platforms. You may need to spend some time exploring different blogs and tags to find what you're looking for.
Perhaps a plot where the Holy Grail War from Fate Stay Night gets disrupted by the arrival of some key players from Fate Apocrypha. This could lead to new alliances and rivalries. Like Shirou from Fate Stay Night having to team up with a character from Fate Apocrypha, say Sieg, to face a common enemy that is after the power of the Grail in both their worlds.
One idea could be to have the Servants from Fate Grand Order interact with the events and characters of Fate Zero in unexpected ways. For example, Mash could end up in the middle of the Fourth Holy Grail War and try to prevent some of the tragedies from happening. She might team up with a young Kiritsugu, using her knowledge from the future to guide him. Another could be that a Grand Order Servant like Artoria Alter gets involved in the war and changes the power dynamics completely, as her dark and powerful nature would be a wild card in the already complex Fate Zero setting.
One really interesting Fate x Nanoha fanfiction is 'A Meeting of Magical Worlds'. It combines the unique magic systems of both series in an engaging way. The characters are well - written, staying true to their original personalities while also developing in new and exciting directions within the fanfiction's plot.
One popular 'Villain x Heroine Fanfiction' is 'The Redemption of the Dark Lord'. In this story, the villain starts to question his evil ways because of the heroine. Another is 'Love in the Shadows' where they are forced to work together. And 'Unexpected Alliances' which shows how their relationship evolves from hatred to love.
Rin Tohsaka from Fate Stay Night and Astolfo from Fate Apocrypha. Rin is a serious and intelligent mage, while Astolfo is a carefree and somewhat eccentric Servant. The combination of Rin's no - nonsense attitude and Astolfo's wacky behavior would make for some really interesting and humorous exchanges.
Rider from Fate Zero and Ozymandias from Fate Grand Order could be a fascinating pair. Both are powerful and prideful kings. They could engage in a battle of wits and power, or they could find common ground in their kingly natures. Maybe they would discuss their different kingdoms and the ways they ruled, and their conversations could be filled with grandiosity and philosophical debates about leadership and power.
The appeal lies in the combination of two beloved series. Fans of both Fate and Nanoha get to see their favorite characters interact in new ways. It's like a crossover of two different magical worlds.