The most interesting part is seeing the different power systems interact. Nen from HxH and Quirks from BNHA are so different. For example, how would Gon's Jajanken deal with a villain's powerful Quirk? It creates a lot of unique battle scenarios.
There's 'Quirks and Nen: A Crossover'. In this fanfic, the power systems of both universes collide in a really cool way. It dives deep into how characters from HxH would adapt to the BNHA world and vice versa. It also has some great plot twists that keep you hooked.
The Bnha universe's reaction would be mixed. Those who are into complex stories, like Todoroki, might study the relationships in Hamilton fanfiction. But others, like Mineta, might not be interested at all because it doesn't have much to do with his usual interests. Overall, it would depend on the individual characters' personalities and interests.
Some would be inspired. Characters like All Might might see Peter Parker's journey as similar to his own in some ways. Both are trying to do good in a world full of challenges. All Might could appreciate Peter's determination to protect his city despite having a normal life outside of being a hero.
For heroic - centric fanfictions, they would likely be disdainful. Since they oppose heroes, seeing stories that glorify heroes would annoy them. They might scoff at the idealized versions of heroes in those fanfictions.
I don't think so. There haven't been any official announcements or indications that the HxH manga will continue. But you never know, maybe there'll be a surprise in the future.