In the fanfiction, a big challenge could be the jealousy or interference from other characters. There might be other people who have feelings for Naruto or Hanabi. For Naruto, Sakura might still have some unresolved feelings, and she could potentially cause some problems for their relationship. For Hanabi, there could be members of the Hyuga clan who oppose her being with Naruto, thinking that he is not a suitable match for her in terms of clan traditions and politics. All these factors combined would create many challenges for Naruto and Hanabi to overcome.