So glad someone's asking about Ichigo/Hichigo content—that dynamic is my absolute favorite part of 'Bleach' fanfiction, and the internal conflict angle gives writers so much to work with. I've been trawling through fics for years, and I keep coming back to 'Internal Affairs' by FahR on FanFiction.net. It's old but it's the classic for a reason. The way it handles their forced co-existence shifting into reluctant understanding, then something far more intense, really nails the tension from the manga while taking it to a much more intimate place. It’s a slow burn that makes the eventual merging feel earned and strangely romantic.
Another one that sticks with me is 'Reign of the King' by Tiro, which is more action-heavy but the emotional core is still their evolving relationship. It treats Hollow Ichigo not just as a power-up but as a separate entity with his own desires, which is exactly what I look for. A newer author on AO3, HollowWhisper, writes some amazing shorter pieces under the 'Shirosaki' tag that are all character study and simmering tension—less plot, more mood, but it works.
Honestly, half the fun is using the filters on AO3: sort by kudos for the established giants, but then dig into the 'Complete Works Only' filter and sort by date updated to find passionate new takes. The pairing has a dedicated following, so even the niche stuff tends to be well-characterized.