The fanfiction might also explore the relationship between Bumblebee and Megatron in a more emotional way. Maybe Bumblebee discovers a softer side of Megatron that was hidden under all that aggression. It could be that Megatron has some regrets about his past actions, and Bumblebee becomes the one he confides in. Or perhaps Megatron sees in Bumblebee a potential future for Cybertron that he once hoped for but thought was lost. This could lead to a very unique and unexpected bond between the two characters.
Yes, there are quite a few out there. You can start by looking on fanfiction websites like Archive of Our Own. Just search for 'tfa megatron x starscream' and you'll likely find some interesting reads.
Another idea is that they are both flung into an unknown dimension. In this new place, their usual allegiances don't matter as much. They have to rely on each other to survive. Bumblebee's ability to adapt and Megatron's strength could be the key to their escape. During this time, they might develop a grudging respect for each other and when they return to their own world, their relationship could be forever changed.
Another common theme is loyalty, or rather the lack of it. Starscream's loyalty to Megatron is always in question, and Megatron is always aware of this. It makes for a lot of tension and drama in the fanfiction. For example, in many stories, Starscream will side with the Autobots if he thinks it will help him gain power over Megatron. This shows how tenuous their relationship can be. Also, the theme of ambition runs through a lot of these fanfictions. Both characters are highly ambitious, and this drives their actions and interactions with each other. They are both willing to do whatever it takes to achieve their goals, whether it's ruling the Decepticons or just being the most powerful Transformer in general.
Yes, there are some. You can start by looking on popular fanfic platforms like Archive of Our Own. There, you might find various stories exploring the relationship between TFA Megatron and Orion Pax. Just use the search function with the relevant names.
Power struggle is a common theme. Megatron is the leader and Starscream is always vying for more power. Another theme is loyalty, or lack thereof, especially from Starscream's side.
There's also 'Power Struggles in the Decepticon Ranks' which is really interesting. In this fanfiction, Starscream constantly tries to outwit Megatron, and Megatron is always one step ahead, yet finds Starscream's attempts both annoying and somewhat endearing in a strange way. It's full of action and political intrigue among the Decepticons.
Fanfiction.net is another option. It has been around for a long time and has a wide range of Transformers - related fanfics. Just use the search bar to look for the specific pairing.
One plot idea could be that Megatron and Optimus Prime are forced to team up in an alternate dimension. They have to put aside their differences to face a common enemy that is threatening the very fabric of that dimension. Maybe it's an ancient evil that has the power to destroy all Cybertronians.
In a unique way, their relationship could be developed if they were both trapped in a virtual reality world created by an unknown force. In this world, they have to rely on each other's skills to survive and escape. As they work together, they start to respect each other's abilities more and their relationship changes. They might not become friends, but they would have a different kind of understanding.