Redesigning the first level, suggestions needed

Hey everyone, I have received major criticism about the first level of the game. I understand this and want to make it a better experience for everyone. Im adding lives, enemies, power-ups, a swimming mechanic, and some bonus stages. Right now the only dangerous things I implemented in the first level are spikes and enemies. Any other suggestions would be appreciated. Im also making new bosses because the current ones are pretty rushed.