This idea was kinda random, just something that popped into my head while at work. You've been able to link Marlix with an invisible bat, making it so that any damage dealt to the bat is transferred up to Marlix! Can you do that with a Guardian and an invisible dolphin? You could even make it so that there's a rarer chance for an Enraged Elder Guardian to spawn out in the ocean! Having some sort of random threat from the ocean would be cool, and since dolphins are already coded to approach humans this seems quite doable (from an outsider's perspective, I admittedly do not have much experience in coding).
Just noticed the note at the top of the suggestions forum, so I'm gonna repost this in the mob suggestion area