You can try some popular fanfiction websites like Archive of Our Own (AO3). It has a vast collection of fanfictions from various fandoms, including ones that combine Star Trek, Buffy, and Xander. Just use the search function and type in relevant keywords to find what you're looking for.
One plot idea could be that Xander gets transported into the Star Trek universe. He could use his knowledge from the Buffyverse to help the Star Trek crew deal with some supernatural - like threats that they're not used to. For example, a creature that is similar to a demon from Buffy but with a sci - fi twist.
One idea could be that Buffy and her team are transported to the Star Trek universe. Maybe they have to fight vampires that have been affected by some sort of alien technology. Another could be that a Star Trek crew member visits Sunnydale and has to adapt to the whole demon - slaying thing. They could end up helping Buffy with some high - tech knowledge from the future.
In Buffy and Xander lemon fanfiction, the characters' emotions play a big role. Their long - standing friendship and the underlying tensions are often magnified. It's not just about the physical aspect. The emotional build - up to their more intimate moments is crucial. Also, the setting might be important. For example, a dangerous situation in Sunnydale could heighten their connection and lead to the more 'lemon' - like scenes.
Since I don't promote or encourage lemon (explicit) content, I can't really say about the common themes in such fanfiction. But in general, in normal Xander and Buffy fanfiction, common themes include their friendship, their shared experiences in fighting vampires and other supernatural beings, and the growth of their relationship over time.
There are also some niche fanfiction communities dedicated to specific crossovers. You can try searching for Buffy - Star Trek crossover forums or groups on social media platforms like Reddit or Facebook. Members of these groups often share and discuss their favorite fanfictions. Additionally, Tumblr can be a good source. Some Tumblr users create and reblog fanfiction, so by searching for relevant tags, you might find some great stories.
One great Buffy the Vampire Slayer and Star Trek fanfiction could be one where the slaying skills of Buffy are combined with the technology of Star Trek. For example, a story where Buffy has to fight vampires on a Star Trek - like spaceship. Another could be a crossover where some of the Star Trek characters end up in Sunnydale and have to deal with the demons there, using their own unique skills and knowledge. There are also ones that might explore the idea of a character from Buffy being part of the Starfleet or vice versa.
Then there's 'FanficWizardX'. This author likes to put Xander in unique situations. For instance, they might send him to an alternate dimension where he has to fight different versions of vampires and demons. Their creativity in plot - making makes their Xander fanfictions stand out.
It can seem strange at first. However, if we think of it in terms of the franchise's influence on the fanfiction and vice versa, it's not so odd. The official Star Trek has inspired a vast amount of fanfiction. And in a way, the existence of that fanfiction is like a feedback loop that the franchise can 'observe'. So, while it's an odd concept literally, it makes more sense when considering the relationship between the two.