Using Suggestion to kill an NPC is a very jank way to accomplish that, but, if players actually, you know, work together, it's doable. Use it to send him into an alley so the rogue can assassinate him. Fucking half the classes learn Hallucinatory Terrain. Use that to hide the cliff, then have him walk over the edge. I think people forget a lot of the time that multiple people can collaborate on a single task. Every table I've ever played, people just had the person best suited to the task do the thing, instead of thinking through whether multiple party members could collaborate, even if just to set up the doer for best odds of success.
Of course, but like you said, that involves multiple players working together. As it happens, though, the Hallucinatory Terrain example could in fact be accomplished by one player, since HT doesn't require concentration, but you'd need to determine if you really want to burn two spell slots in a very convoluted manner to simply have a character fall to their death. Either way, I'm personally willing to give more leeway to players that actually come up with some kind of creative plan where they work together to accomplish a goal, as opposed to a single player attempting to pull a fast one on me.
except having someone walk forward is perfectly achievable, but not reasonable. just as the initial example being "just hand over the item". if there's isn't enough common sense existing that anyone is willing to hand over his most treasured possession to some rando, "walk forward" till gravity does it's job is even further removed in the causality chain. and if said DM would wanna argue that's not comparable he's an even bigger idiot. you either have an NPC loose all logic, reason and even emotion, or you don't.
pretty much anything is "achievable" in less than 25 words like "hey, make me the new king for a minute" or "put your thumb up your butt", there's not damage at all, so the rules allow it. but it's hardly reasonable.
Which is why I have an issue with the wording change to "achievable" in that it's far too broad for such a low level spell. Handing over an item is certainly a simple task that anyone can do, but if it's your most valuable possession, it doesn't seem logical that you'd be willing to just give it without question, especially if you think it could cause great calamity if you do so. On the other hand, if there isn't at least some leeway, then the spell becomes effectively worthless. That's DM fiat for you, I guess.
As to your examples, if you had an NPC currently facing a cliff and suggested "walk forward and don't stop," I would rule that they'd still stop when they reached the edge because they would realize they were about to hurt themselves. Suggestion isn't total mind control, after all, so they still maintain their faculties, including a sense of self-preservation. I'd also rule that "make me king for a bit" is also not achievable due to the fact that actually transferring the kingship to a new person involves a lot more than just one person's say so, even if they are the king. Or to put it another way, maybe the suggestion would work on the king, but good luck getting anyone else in the kingdom to believe you, potentially figuring out something's up with how the king is acting around you. Even if you attempted Mass Suggestion, that's still only twelve people maximum, but it's also a sixth-level spell with a longer potential duration, giving you more time to make things happen the way you want them to. At that point, you can already Charm/Dominate Person as well, so I don't see why you'd try to weasel a plain ol' Suggestion when you have access to the better spells you're trying to replicate the use of. Saving your high-level slots, I suppose, but depending on what you're trying to do, I'd probably rule you'd need to use those better spells.
In the interests of not getting bogged down in another back and forth that I already did a year ago, let's just leave it at "Suggestion works as well as your DM will let it."
