There are fanfictions where the advanced technology of Stargate is introduced into the Smallville universe. For instance, imagine the Langarans sharing their medical knowledge with the Smallville doctors. This could lead to some interesting character development as the Smallville characters have to adapt to new ways of thinking. Also, a crossover could explore how the alien races from Stargate would react to a being like Clark Kent with his superhuman abilities.
Some of the best Smallville and Stargate SG1 crossovers involve character interactions. For instance, Lex Luthor could get involved with the NID from Stargate SG1. This could create a whole new dynamic of power - hungry characters vying for control over alien technology. It makes for a really exciting read as the two universes collide in unexpected ways.
Another way it blends them is through the use of technology and powers. The advanced tech of Stargate SG1, like the Stargate itself or the zat guns, can be contrasted with Clark's superpowers in Smallville. Maybe Clark has to deal with a situation where the Stargate technology is malfunctioning and his powers are the only way to save the day. It's this kind of mixing of elements that makes the crossover so interesting.
First, the settings can be merged. For example, the Stargate could be placed near Smallville, so the two worlds collide. Second, the characters' abilities and skills can be combined. Clark's super strength and flight could be used in Stargate - like adventures. Third, the storylines can cross. The Kryptonian mythology and the Stargate exploration themes can be intertwined, like finding a Kryptonian - related object on an alien planet through the Stargate.
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 ability to reimagine character relationships is also unique. Characters who might not have interacted in their original shows can form new alliances or rivalries in smallville crossover fanfiction. For instance, a character from Smallville could have a completely different relationship with a character from 'Supergirl' in a crossover. This gives fans a chance to see their favorite characters in new lights and explore different emotional arcs.
I really like 'Crossroads of Krypton and the Hunting Life'. In this fanfiction, the writers did an amazing job of bringing in the mythology of Smallville and Supernatural. Dean and Sam Winchester end up in Smallville and get involved with Clark's world. It's full of action and some really cool character moments. It shows how the different powers and skills of the characters from both shows can work together or clash in interesting situations.