To continue the HxH manga after the anime, check manga subscription platforms or visit your local comic store. They should have the volumes you need to pick up where the anime left off. Also, some online manga reading sites might have it available.
The Hxh anime ends at a specific point in the manga that's determined by the production decisions. It's not a straightforward mapping, but you can find out by comparing the storylines.
You can also check out Wattpad. It has a variety of fanfiction including some for HxH. While it has a broader range of content including non - fan - based stories, if you search specifically for HxH fanfiction, you might come across some interesting reads. However, the quality can vary more compared to AO3 and FanFiction.net.
One great 'HxH Illumi x Reader' story could be about the reader being a new hunter and Illumi taking an unexpected interest in them. Maybe he's intrigued by their unique nen ability or their fearless attitude towards the dangerous tasks hunters face.
From what I can assume, if it's like other similar works, it may have a unique power system. For example, characters might possess special abilities or powers that are related to their identity in the school or the world they are in. This could lead to battles or competitions among the characters, which is a common and exciting element in such stories.