I have to disagree with you on adding 24-72 hour World spawn HNM's.
Other than what everyone is saying about botting, time sink, drop rates, boring, mpk, etc etc..
You said something along the lines of "they could offer the same gear that you would get in an instance" and that they would "add content to the game".
I'm assuming that being an HNM would mean that they would require a group of people to fight, correct? Why would anyone want to camp around for up to 3 days for a chance to claim a NM, that has a small chance to drop gear? Especially if the same gear or equal gear is dropped in an instance run where there is no competition, can be completed on your own time, and you can develop a meaningful strategy opposed to just claiming the pop then tank n spank?
People will always take the route of least resistance, which in my opinion would be doing an instance run over camping NM's. Which would leave this "New Content" untapped by the majority of people. I'm sure some people would still camp it, but why would a party of 8 people camp an HNM for days over doing an instance run?
Times have changed, people play multiple games, have lives, and many don't want to spend their lives rotting in front of a PC over some digital gear that will eventually be obsolete. Pending this games survival, there will be level cap raises, expansions, new classes, jobs and all of that will make these HNM drops pretty useless.
I'm all for challenging and engaging content but I am certainly not for a "who can claim" "who can bot" "who can sit here the longest" competition.
As someone else stated, fighting for a claim does not make something difficult. Nor does sitting there for countless hours basically doing nothing while waiting for a respawn. Reading most, if not all, the posts in this thread it's clear that very few people believe this should be implemented into FFXIV. I am also into making these types of things instanced with difficulty.