Yes, there might be. The 'The Owl House' has a large and creative fan base, and fanfiction often explores various 'what - if' scenarios. However, the content can vary greatly in quality and adherence to the spirit of the original show.
Another possible main character could be a rival collector. They compete with the main collector for the rarest and most valuable items from the Owl House. This rivalry could create a lot of tension and interesting plot twists, as they both try to outwit each other to get their hands on the most coveted collectibles.
Fanfiction.net could also be a place to search. There are likely some stories there that might feature this scenario. Additionally, you could try searching on Tumblr. Sometimes fans share links to their favorite fanfictions there, and you might find one about Luz being pregnant through Tumblr's search function.
One popular theme is friendship. Amity and Luz's relationship grows from a somewhat rocky start to a really strong friendship. Another theme is self - discovery. As they face various challenges in the Boiling Isles, they both learn more about themselves. Magic is also a big theme, as they use magic in their adventures together.
Well, it's hard to say who the 'best' are, but some authors have really stood out. There's this one author who goes by the name 'MagicWriter123'. Their stories often have really detailed descriptions of Amity and Luz's relationship. Another good one is 'BoilingIslesFan'. They seem to have a great understanding of the show's lore and incorporate it well into their fanfics.
One interesting 'owl house collector fanfic' story could be about a collector who discovers a rare artifact from the Owl House world that has magical powers. This artifact then leads the collector on an adventure through different realms in the Owl House universe, encountering various characters along the way.
If Luz was Eda's daughter, her relationship with King would be more like that of siblings. King might be a bit more protective of Luz or jealous when she gets more attention from Eda at times. And Luz would probably be more bossy towards King in a sisterly way.
Well, another option is to check out some dedicated fanfic communities on Reddit. There are subreddits related to the show where fans might share their 'collector luz fanfic' or talk about their favorite ones. You could also try searching on Wattpad, though the selection might be a bit more limited compared to AO3 and Fanfiction.net.