Nobody makes up custom moves in order to 'patch a hole' in action resolution, no such holes can arise by the nature of the construction of the Dungeon World rules. Custom moves are a POSSIBILITY which could be exercised by the table, as a way of portraying some different sort of character archetype, or some kind of ability that simply isn't already covered that the participants want to emphasize. Again, its not patching a hole in the action resolution system. It would be more like adding a new class ability in D&D, and done for largely the same kind of reason.
The other thing that GMs sometimes do (well, often I guess overall) is construct custom moves for THEMSELVES, or as 'SPECIAL MOVES', which are a category that are 'passively activated' by PCs. I think the "Outstanding Warrants" move was discussed in the Bloodbath thread. This is a move which a player triggers when they enter a town where they might be wanted. GMs could make up new moves of this sort. I think
@Manbearcat in some thread mentioned creating a custom move for a PC in an AW game where the move was triggered at some interval and the associated roll of dice helped determine what was up with the character's holdings. The GM could instead have just made a GM move to say "You made some money this month" or "Raiders are active in the South Fields" or whatever. I guess you could characterize these as 'subsystems' and 'new rules', but they don't rise to any higher level of such than, say, using the 2e DMG "make a custom encounter table" rule to customize encounters in a specific area in a game. Hardly anyone would call that a house rule, the DMG says to do it, and how to do it, its just backstory.
Nope, I suggest playing Dungeon World, there's no such need in that game. When a player declares that his PC is taking an action, that action HAPPENS. Either the table rules that the action cannot succeed (and generally this determination often is made before the declaration is formally made since it is advantageous to insure that all players have the same fictional position in mind) and it fails automatically, or it is possible and either a move is triggered and a roll made, or the action succeeds automatically. This covers ALL POSSIBLE ACTIONS in Dungeon World (and other similar PbtAs generally speaking).
So, if your character is 'plunging a toilet', I'd first of all consider this too trivial to need to adjudicate or even frame into a scene, but lets assume somehow it is a critical action. So, can the character succeed? I'd say its quite possible! Does this trigger any existing move? I'm not aware of one in Dungeon World, though 'Defy Danger' is fuzzy enough that basically you could trigger it for most things, so the table will have to decide here, is this so hazardous that DD is needed? If not, then it just succeeds! I mean, frankly unless there's a bomb in the crappy that might be set off or somesuch, I favor the poop goes down the hole, lets move on! But lets assume there IS a bomb in their, then DD is your go-to move, with maybe INT to disarm, DEX to shove it down the pipe before it can explode, etc.
Same with the mile-long tunnel. I mean, this sounds like a more involved kind of a situation, but it might be handled as a single action depending on where the GM and players want to focus their attention (IE its part of a large siege where lots of different actions are happening). Again, its probably just a matter of context as to what happens. If the lead PC on that job is a dwarf with a mining backstory, he probably just succeeds! If not, or if there's obvious dangers involved, then DD is again your friend, maybe with +WIS in this case.
Thus we can see, this is a very 'complete' game, there is an established mechanical approach to resolving ALL 'plot issues'. And that's really the key to this sort of game, that its not about simulating the results of taking certain actions. Its about determining what the development of that part of the narrative will bring. Does the dwarf undermine the castle wall? Play to find out! Would 'real' dwarves in a 'real situation' succeed? I dunno, it is a fantasy world, the question isn't meaningful. I know that there are at least a couple of story possibilities and the interesting ones are going to be picked between, some dice thrown and one chosen. It works every time for all plots, because plots are all the same at heart, they all involve fictional elements and things that need to be decided, which is what the rules do.
As for some sort of 'fiat', you clearly need to play one of these games, badly, under a reasonably skilled GM with experience in running it. There's no 'fiat' involved. You are reasoning from an idea of an all-powerful game master who is charged with inventing all of the fiction and authority to make it whatever they feel like. This situation doesn't exist in a Dungeon World game. Even when the game calls on the GM to make something up (and it does this OFTEN) the things that are made up have to meet fairly specific criteria. That does still leave infinite possibilities, but because many of those criteria involve feedback from players, directly or indirectly, and sometimes players even get to say fiction beyond their PC, it takes on a very different character than what happens in a D&D game. You can repeat endlessly that it is just the same old GM fiat, but you will endlessly be wrong!
