Yes, there are some out there. You can start by looking on fanfiction websites like Archive of Our Own or FanFiction.net. Just search for 'Transformers Prime Jack x Solus Prime' and you'll likely find a variety of stories with different takes on their relationship.
Adventure is a big theme. Since Jack is often thrust into the world of the Transformers in the series, fanfictions tend to continue that trend. He and Solus Prime might go on quests or face threats together. Also, themes of mutual respect and understanding between their two species are quite common as they learn from each other.
A plotline could be Sentinel Prime getting corrupted by dark energy and Optimus Prime having to fight him to bring him back to the light. It would be a battle of wills and also a test of their long - standing friendship.
Definitely not. In the Transformers universe, Optimus Prime represents leadership, honor, and protection. Associating him with a child in an improper way in fanfiction is wrong. Fan creations should promote positive values. We can have fanfiction where Optimus Prime is a mentor or protector of children in a proper, non - inappropriate way, but not the other way around.
It's difficult to determine its popularity. Just because we haven't seen it widely promoted doesn't mean it isn't popular among a certain group. It could be that it has a small but dedicated fan base that really enjoys the unique concept of the mc being forced to drink 'liquid x'. Maybe it's new and just starting to gain traction, or it could be an older title that has been overshadowed by more mainstream light novels.
Fanfiction about them can explore different universes and scenarios. For example, what if they were raised together? How would their relationship be different? It gives fans a chance to play with the established lore and create new and exciting stories that the official Transformers storylines may not explore. It also allows for deeper exploration of their motives and personalities.