One great hashirama senju crossover fanfiction is 'Hashirama in the Modern World'. It's interesting as it shows Hashirama adapting to modern technology and society. His confusion and awe at things like cell phones and cars make for a really fun read.
There's a fanfiction called 'Madara and Hashirama: A Different Path'. This one takes a unique approach as it starts from an alternate universe where Madara and Hashirama make different choices early on. It's fascinating to see how their relationship evolves in this new setting, and how their love/hate relationship still remains at the core, but with different manifestations.
One common theme is their friendship. Despite being on opposite sides at times, many fanfictions explore the deep - seated bond they had. Another theme is redemption. Madara, especially, is often the subject of redemption arcs in fanfiction. Also, the theme of sacrifice for the greater good is seen, as both characters in the original story were willing to make sacrifices for their respective villages.
I'm not sure if there is an actual 'Harry Potter is Hashirama Senju fanfiction' out there. But if it existed, it might be a really wild crossover. Maybe it would involve Harry Potter somehow getting the powers or wisdom of Hashirama Senju. It could be about Harry using Hashirama's wood - style jutsu in the wizarding world to fight Voldemort or other dark forces. Or perhaps it's about the two characters' parallel growth and leadership experiences in their respective fictional universes.
You can start by looking on popular fanfiction websites like FanFiction.net or Archive of Our Own (AO3). These platforms have a large collection of fan - made stories, and you can use their search functions to specifically look for hashirama senju crossover fanfiction.
One common theme is the bridging of different eras. Since Sakura is from a much later time than Hashirama, fanfictions often explore how their different knowledge and values interact. For example, Sakura's modern medical knowledge vs Hashirama's more traditional ninja skills.
I think it has the potential to be popular. In fanfiction, unique family relations like Naruto being the grandson of Madara and Hashirama can open up a plethora of new plotlines. For example, it could change Naruto's relationship with Sasuke, as their clans' history would be even more intertwined. Also, Naruto might inherit some special abilities from his supposed grandfathers, which is always exciting for fans to explore.
Well, I'd first check out the big fanfiction platforms. Archive of Our Own has a large collection of various fanfictions. You might need to dig deep into the search results. Sometimes, less - known fanfiction communities or Naruto - Harry Potter crossover forums might also have such a story. You could also try reaching out to other fans who are into both Harry Potter and Naruto to see if they know of any existing fanfictions with this concept.
It could be Orochimaru. He's always been interested in the power of the Senju clan and the first Hokage. With Hashirama being reincarnated, Orochimaru might see this as an opportunity to either steal his power or use him for some dark experiment.