AO3 Harry/Wednesday This search excludes works that require good Malfoys or Snape. I’ve seen attempts to redeem them based on this pairing be reasonably done, but it is a stretch.
A03 Harry Addams Family xovers This search is primarily based on having Gomez/Morticia as a pairing.
Fanfiction.net has no good way to search for crossover pairings. Here are the crossover collections I have found.
FFn HP/AF XOver Seems to have the most results