I believe there are ways to make it fun and diverse while still having the randomness uniform and fair. For example, when you roll a six for the knifes, there could be six knifes every 10 seconds, but if you roll a one, a knife could appear every 3 seconds! Stuff like that. I think the fish is actually fairly good at this, since I'd say it's about as difficult no matter the jump height – but it still changes how you approach the enemy!
It's for you to consider though, don't forget it's your game ;)! So do what you think is going to be the most fun.