Naruto could be one of his allies. Naruto has this knack for seeing the good in people and would probably be willing to give Obito a chance to redeem himself. Also, Sakura, with her skills as a medic - ninja, could be an ally. She might help heal Obito if he was injured during the war and see the potential in him to be a force for good.
To develop Hashirama's ghost, we could give him a sense of purpose. He could be a protector of the village in the afterlife. So, when he senses a great danger approaching the village, he decides to reach out to Naruto. He could have a deep connection to the natural world even as a ghost, and his powers could be related to manipulating nature spirits. He could teach Naruto how to communicate with these spirits and gain their assistance in times of need.
The sad story of Obito and Rin is a key part of the Naruto narrative. Obito was a young ninja with big dreams and a huge crush on Rin. When Rin was seemingly killed in front of him, it shattered his world. He thought that the ninja world was full of pain and betrayal. This led him to be influenced by Madara. He became one of the main antagonists, all because of his love for Rin and the unbearable pain of losing her. His journey from a hopeful young ninja to a vengeful villain is a tragic consequence of Rin's death.
The main character is Obito without a doubt. The other main character in the 'obito love story' could be a character from a different clan. Their different backgrounds would add more depth to the story. For instance, if Obito is from the Uchiha clan and the love interest is from the Hyuga clan, their relationship would be full of cultural and clan - related tensions, making their love story all the more interesting.
Fanfiction.net is also a good place to search. There are many Naruto fanfictions there. However, the search functionality might not be as refined as AO3. You may need to dig a bit deeper. Browse through the Naruto category and look for tags related to gay relationships between Naruto and Hashirama or just skim through the stories to find what you're looking for.
There could be. The Naruto fandom is vast and has all kinds of creative fanfictions. However, this particular concept of Sakura being pregnant with Hashirama's something (it's not clear what exactly in this title) is a bit out - of - the - ordinary. It might be difficult to find, but you might get lucky if you dig deep into the Naruto fanfiction archives on different platforms.
In such a fanfic, perhaps Obito could use his Sharingan powers in new and unexpected ways after the war. He might start to see the world differently and strive to make amends for his past actions. He could use his abilities to help rebuild the villages he once tried to destroy, like Konoha.