Help with a runaway plotline

[Weave]

I recently started a 5E D&D campaign (a level 2 one-shot) that I thought would end then and there. When the session ended and it became apparent they might want to reinvest in these characters in the near future, I gave them a map with the details to an ancient fallen dwarven empire treasure vault without much thought on the details. We had a few more sessions that took considerably longer than I expected, but were fun all the same. Simultaneously, the players were joking about how bummed they'd actually be once they found the treasure, signifying the (potential) end of their adventure. It's about here when I decided to go all the way and may have, inadvertently, jumped the shark, so to speak.

I decided that it wasn't just a map to a dwarven treasure vault of some ancient empire, but a map that led to one of four magical "keystones" that would then be used to unlock the vault. To further overcomplicate things, I decided to add a BBEG to really motivate them: a dwarven wizard who existed during the old dwarven empire who has been using powerful reincarnation magic to keep coming back in different forms with the intentions of unlocking said vault for his own nefarious deeds (to eventually shed this mortal coil and ascend to godhood). Today, my players captured the first of the four keystones (now all level 6) and one of them wondered why they don't just hide this one away and ditch the rest of the quest entirely. It was something that, in my hastiness to create a plotline, I overlooked, and while I think I could run with that, I'm not sure I have the capacity to shift gears so suddenly (regardless, the others decided the treasure was more valuable anyways and wanted to continue, but the other player seemed sort of removed and not as into the quest). Should I try and tweak the plot even further to come up with an explanation as to why they'd want to pioneer onward? Or should I let them run with the offshoot hideaway idea, despite not having very many ideas for it.

WHAT'S A DM TO DO?

EDIT1: I like to think of myself as a fairly accommodating DM, and don't want to give off any railroady vibes - I honestly prefer my players steer the plot in whatever direction they like, I guess I'm just not entirely sure how to roll with the idea of completely reversing what I had intended.

EDIT2: I also don't have a great in-game explanation as to why they just found the map, and why this great dwarven BBEG with literal centuries of experience hadn't come across it beforehand. I could chalk it up to fate or dumb luck, but no one in the game has addressed this yet so maybe I shouldn't even bother?

[sparkletwist]

If there are four magical keystones, then maybe the evil wizard simply hadn't gotten around to that one yet. There are three other ones out there, all presumably hidden away with their issues to get them, and maybe the wizard was simply occupied with the others, so the one they happened to find was simply the one that the wizard had been paying the most attention to.

You could even ratchet up the tension (and shorten the game a bit, if you want) by forcing a confrontation between the wizard and the party, by saying that the wizard has already found 1-3 of the other keystones, and is now trying to grab the fourth. That might also impress upon the group the basic impossibility of simply hiding the fourth one away, or, at least, push them enough in that direction that it feels like that's the "right choice" without being railroady.

Another idea that might help defeat the "just bury it" idea is that the map is magical, and it shows the location of the keystone wherever it is rather than simply being a map to the location where they found it.

[Steerpike]

I like sparkletwist's map idea a lot.

Have they met the wizard yet? Does he know they have the keystone? If so, he could easily be scrying them when they hide the keystone (according to the spell they do get a save - but he could repeat the spell on different party members - give him a scrying device if he doesn't have the spell slots). If he gets even an approximate location then locate object can pretty easily do the rest. Alternatively, he could try and use detect thoughts on someone. Or capture someone the PCs care about and hold them hostage unless the keystone is given to him. Or send out minions to start tearing up the countryside looking for the thing, knowing the PCs hid it somewhere.

[Weave]

If there are four magical keystones, then maybe the evil wizard simply hadn't gotten around to that one yet. There are three other ones out there, all presumably hidden away with their issues to get them, and maybe the wizard was simply occupied with the others, so the one they happened to find was simply the one that the wizard had been paying the most attention to.

You could even ratchet up the tension (and shorten the game a bit, if you want) by forcing a confrontation between the wizard and the party, by saying that the wizard has already found 1-3 of the other keystones, and is now trying to grab the fourth. That might also impress upon the group the basic impossibility of simply hiding the fourth one away, or, at least, push them enough in that direction that it feels like that's the "right choice" without being railroady.

Another idea that might help defeat the "just bury it" idea is that the map is magical, and it shows the location of the keystone wherever it is rather than simply being a map to the location where they found it.

I like the idea of the wizard already having one of the keystones, and maybe all 3. The map is already magical, so having it trace the location of each stone is definitely an awesome idea.

Have they met the wizard yet?

Yes. He appeared before them when he orchestrated their inadvertent access to the forbidden section of a prominent library in a large city, informed them who he was, and asked them to join his cause (there's also an elf wizard in the party who has a similar wish to understand and master magic, so I was playing it up for his character). When they declined and moved to attack, I had him set the area around him ablaze, consuming his own form (since he reincarnates) and much of the library, as a power move. The elf in the party also texted me in secret saying although he declined, he did so only to stand with his allies at the time, but in reality wishes to re-establish contact so that he might reconsider, so, potentially a very interesting turn of events.

Does he know they have the keystone?

He doesn't know just yet, but he will soon enough. I like all the aforementioned ideas as well. He's has a bit of a mad scientist bend to him, so aberrant monstrosities tearing up the hillside sounds right up his alley.

Thanks for the help! Right now I'm going to decide if he should already have all 3 stones or not.

[Ghostman]

Be sure to also make the stones indestructible. Or alternatively, affix sufficiently catastrophic consequences to their destruction so that players won't be tempted to just try and break this one stone as another easy way out.