Tatsumi is very likely to be involved as he has a significant relationship with Esdeath in the original story. Her subordinates in the Jaegers might also be part of the fanfic. They could be shown as being protective or having mixed feelings about her pregnancy.
It might not be so straightforward. While Esdeath has the potential to be the antagonist, there could be other forces at play. For example, there could be a third party that is manipulating both Tatsumi and Esdeath for their own gain. In this case, both Tatsumi and Esdeath could be seen as victims of this greater evil, and they might have to team up against this new threat. However, if we consider the traditional view, Esdeath's actions and beliefs are in direct conflict with Tatsumi's, so she remains the likely antagonist.
The Akatsuki could still be major antagonists. Their goals of collecting the tailed beasts would put them in direct conflict with Naruto, especially if he has the added power from his relation to Esdeath. They might see him as an even greater threat.
If Naruto is Esdeath's brother in fanfiction, they could embark on adventures across different dimensions. Since Naruto has experience with his own world and Esdeath with hers, they could explore new lands together. They might encounter strange creatures and powerful enemies. Along the way, they could learn from each other's fighting styles and abilities, Naruto teaching Esdeath about chakra and she teaching him about her own brand of combat techniques. And they could also be on a quest to reunite their fragmented family or uncover a long - hidden family secret that has the power to change their worlds.
In certain fanfiction works, Tatsumi is indeed made into Esdeath's pet. This can be a way for writers to explore darker or more extreme versions of their relationship. It might involve Esdeath treating Tatsumi in a demeaning way at times, or it could also lead to a strange form of affection developing between them as the story progresses. It really varies from one fanfic to another.
Esdeath's demise in the manga is quite intense. It's a culmination of her actions and the circumstances she finds herself in. To get the full impact, you really need to delve into the manga pages.
Some characters might adapt well due to their own desires for power. For example, Sasuke could see the peerage system as a means to gain more strength to achieve his revenge. He would likely be drawn to Esdeath's strength and might be willing to submit to her in order to learn from her combat skills.