In many TWD OC (Original Character) fanfics, Negan's overconfidence could be seen as a weakness. His cocky attitude might make him underestimate his opponents, especially new and unpredictable OCs. This could lead to him making tactical mistakes in battles or negotiations.
Well, Negan's death in the TWD comics is a result of multiple factors. There's a final confrontation where his enemies unite against him, and he's just outnumbered and outmatched.
If Negan's overconfidence is the target, an OC could play on it by feigning weakness or incompetence. Make him think they're an easy target, and then when he least expects it, strike back hard. For example, the OC could pretend to be a scared survivor begging for mercy, but then when Negan gets close, they use a hidden weapon to attack.
Yes, there are likely some out there. You can try searching on fanfic websites like Archive of Our Own (AO3) or Fanfiction.net. Just type in the relevant keywords and see what pops up.
Being saved from Negan is a big deal because Negan was a major threat in The Walking Dead. His brutality and power made him a feared character. So in the fanfic, it could be a turning point for 'them', a chance for new beginnings or a way to show their resilience.