Since Shepard is able to talk Saren into killing himself and do the same with TIM, yet he/she can't talk like that to the Leviathan turd. He/she may not change the turd's "mind", but at least give Shepard the opportunity to talk big to the thing.
I'd go beyond talking big. It seems like a futile endeavor anyway. It has Shepard's mind in some weird interface in which they control whether or not he/she is released.
What I wish I could've done was tell Hackett about any plans regarding the Leviathan once the war is over. They're too hazardous to leave unchecked, and I would've told him that once the reapers were neutralized, it would be best to destroy any and every artifact they find to blind them once they're no longer useful, quarantine that planet of theirs and let them rot in their water world. Of course, that's no longer relevant since it's not likely that the relay in that system would be restored anytime soon, but still.