"What is in that chest?"
Translation:Kio estas en tiu kesto?
Personally, I don't think of profanity as an "intensifier" in any language. To me it's more an indicator of the user's inability to express themselves otherwise. That said, I feel that your first example, given Esperanto's penchant for literal interpretation, might be severely misunderstood to suggest that the box is either made of or used for feko. The second would probably work better.
There is also the root diabl- which I've seen utilized as a sort of intensifier. "Kio estas en la diabla skatolo!" but mostly, when trying to suggest any level of "badness" -aĉ- and fi- do the job well enough.
However, I have to admit that I'm not anywhere where I can hear the language spoken outside of my house. Thus, you should freaking take my fecal suggestions darned advisedly.
"To me it's more an indicator of the user's inability to express themselves otherwise"
Maybe in terms of excessive use (however you'd define that), but otherwise I'd think that's a narrow-minded opinion.
Would you rather someone, after say, stubbing their toe, "Ah that ❤❤❤❤❤❤❤ hurts!" with "Ah, I find this to be painful at an excessively intense level!"? Sounds ridiculous.
Sudden pain has been shown to lower the mind's ability to form such coherent, wordy sentences. :D
I've managed to go through 60 years of stubbing toes and catching fingertips in doors and having hot oil spit at me and etc without having to resort to profanity on that level. Usually a mere "blast!" or even an occasional "damn" escapes me. Even the time that I found that, yes, that was a bullet that had hit me, I believe I let go with a withering Crimeny or perhaps even a shoot! (knowing my inborn penchant for puns, I'm hoping that it was the later) leaving those around amazed at my apparent "calmness" given the situation.
I'm not saying that you have to do as I do, but knowing that I can do, and have done, it without saying things to increase some kid's vocabulary just means that it can be done.
That all said, I will, and, again, have, express admiration at profanity used well and artfully. (note my use of watered down execration, above, done for humorous effect.) Just not at such usage intended to shock, titillate, or replace more appropriate terms of whichever part of speech one elects to ineffectuate.
If you wish to cuss to your heart's content, go ahead, don't let me stop you. This is a personal choice for everyone. I've stated where I stand, I don't insist that you, or anybody else, have to follow my choices. I've also noted that Esperanto doesn't have the same format as English does for verbal imprecations. Not that it can't, but that one really doesn't need to resort to such.
Now, you may wish to go to the UEA and see if they still have the little pamphlet: "Knedu min sinjorino!."
I can also tell you about the most amazing fireworks I witnessed from a fisherman once. That was five minutes of unrepeated invective directed at a motor that finally worked after he had had to row his commercial size fishing boat into the slip. The incredible creativity and catharsis he demonstrated made all who witnessed the event stand up and cheer. Even the not-really-English speaking tourists.
In "kion" the -n implies the accusative. There's no action being taken here on anything, and as such, we don't need to denote one that is receiving it. An example that might not be the best but will hopefully get the point across is:
"Li bezonas malfermi la keston." - He needs to open the chest. In this case, someone is opening something. With the "n" on "kesto," we know that the chest is being opened. If you wrote "Lin bezonas malfermi la kesto," the chest would be opening the guy, which is hilarious/disturbing, but wrong in most cases.
"Li estas en la kesto" - He is in the chest. There's no object being acted on, just the dude being in the chest. We don't need to worry about any ambiguity here, so the accusative isn't necessary.
Hope this helps.