It's hard for me to choose one event, but I'm pretty sure the biggest problem will be high fees and slow transactions. As more people start using Bitcoin, what we currently see only from time to time (usually during the bulling market) with $40 fees can become the new normal unless some solution becomes truly popular (I don't think the Lightning Network can succeed in this because it failed to gather enough users over the years). It might be an off-chain solution, but I think it will be something else because many are against off-chain solutions. Perhaps strict legislation against crypto fraud and zero-confirmation transactions for sums below $50 in physical shops will become popular because it's the simplest solution. Maybe something smarter that we didn't think of yet. The biggest risk of a hard fork will be if someone figures out how to make BTC way more scalable but it requires serious changes.