One common theme is the exploration of forbidden love. Since it's slash fanfiction, often the relationships are between characters that might not be expected to be together in the original series, so there's that element of the relationship being against the norm.
One great stargate slash fanfiction is 'The Unexpected Bond'. It explores a unique relationship between two main characters in a really engaging way. The writing is quite good and the plot development keeps you hooked.
One popular story might be 'The Unlikely Alliance' where characters from both universes come together in a unique way. It could involve a Hogwarts student accidentally stumbling upon a Stargate and then the two worlds having to cooperate. However, finding really good ones can be a bit of a search as it's a rather niche combination.
Start by doing some research on both Harry Potter and Stargate. Know the rules of magic in Harry Potter and the technology and different species in Stargate. This will help you create a more believable story. You could also start with a small idea, like a single character from one universe ending up in the other, and then build the story around that. For instance, if Hermione were to end up on a Stargate - enabled planet, how would she use her knowledge to survive? And how would the locals react to her magic? It's all about creating an interesting and engaging scenario that combines the two universes in a unique way.
First, you need to be really familiar with both universes involved in the crossover. For Stargate, know the different gates, the Goa'uld, and the various teams. Then, come up with an interesting premise. Maybe it's a new enemy that can only be defeated by the combined forces of the two universes in your crossover. Keep the characters true to their original personalities.
A Stargate and Battlestar Galactica crossover can also be amazing. Picture the Stargate teams dealing with the Cylons. The fanfiction often explores how the military strategies from both franchises would clash or blend. It usually has a lot of action - packed scenes as well as some deep character - driven moments.
A possible fanfiction could revolve around a joint mission. The races from Babylon 5 and the teams in the Stargate universe come together to face a common threat. Maybe an ancient evil that has the power to destroy both their worlds. They would have to combine their knowledge and resources, like the Psi Corps abilities from Babylon 5 and the Ancient technology in Stargate.
One idea could be that a Sentinel's enhanced senses are used to detect a hidden Goa'uld base that the Stargate team has been searching for. The Sentinel could sense the unique energy signatures or the presence of the symbiotes in a way the normal technology couldn't. Another idea is to have a Sentinel character be accidentally transported through the Stargate to a different planet. There, they have to use their Sentinel abilities to survive and team up with the Stargate team members who are also on a mission there.
The Ministry of Magic would probably oppose it. They are always concerned about maintaining the secrecy of the wizarding world and a stargate could potentially expose it to non - magical beings or lead to unwanted magical interference from other realms.