power ups could be a really fun way to add to the gameplay, or upgrades for the hook and boat. I like the 'pull something of an animal first to save it' idea also. A boss fight could be fun, but difficult with the area shrinking. Maybe if it's something that can swim through the death zone and tries to take over your hook or something?
Yeah I think I'd have to change the game to one where you could push the wave back somehow, so you could increase your time playing. I do like the idea of something disabling your hook temporarily, I'm not sure what I could use as I'd like to keep all the animals peaceful. This thing could have an attack that tangles your hook so you have to avoid, then try and hook it at the right time to beat it. Nice brainstorming!