Another place to look for it is Archive of Our Own (AO3). It's known for its wide range of fanfiction, and there are quite a few 'sonic x rarity' fanfics there. You can also filter the results by rating, length, and other criteria to find the perfect one for you.
A popular 'sonic x rarity fanfic' is 'The Sonic - Rarity Bond'. This fanfic delves deep into the relationship between Sonic and Rarity. It starts with a simple encounter but then develops into a story about how they learn from each other and grow together. The descriptions of the settings are really vivid, making you feel like you're right there with them.
In a Metal Sonic x Sonic Fanfic, the action and adventure aspects can be really exciting. Writers may create new challenges for Sonic and Metal Sonic to face together or against each other. For example, they could be forced to team up to defeat a greater evil that threatens their world. This kind of fanfic also gives a chance to delve into the emotions of the characters. Metal Sonic, although a machine, might show signs of evolving emotions or at least a form of artificial intelligence - based loyalty or rivalry towards Sonic.
A 'sonic x sonic unleashed fanfic' could be really interesting. It might explore new adventures for Sonic in the context of the Sonic Unleashed world. Maybe it shows Sonic facing new challenges or forming new relationships within that setting.
One common theme is unrequited love. Often, Spike has strong feelings for Rarity while she may be initially oblivious or focused on other things. Another theme is growth. Their relationship helps both of them grow as characters, like Spike becoming more confident and Rarity learning to see beyond appearances.
Adventure themes are also popular. The two characters might go on an adventure together, and during this journey, their bond grows stronger. It could be an adventure in a magical land or a quest to solve a mystery in Ponyville.
You can look for it on major fanfic platforms such as Wattpad. There are a large number of fanfics there, and you might be lucky to find ones related to 'rarity x elusive'.
Romance is also a popular theme. Writers explore the relationship between the two ponies in a romantic light. This could involve them realizing their feelings for each other gradually or in a more sudden, 'love - at - first - sight' kind of way.
The themes of power and rivalry are quite prevalent. Metal Sonic is designed to be as powerful as Sonic, if not more so in some cases. So, the fanfic may focus on their power - based rivalry. They constantly compete in speed, strength, and agility. Additionally, the theme of technology vs. nature can come up. Sonic is a natural - born being, while Metal Sonic is a product of technology. This contrast can lead to interesting plot developments.