I don't think this is correct... combat with environmental factors (cold, rain, heat exhaustion, high altitude, etc.) make it harder. Combat in unfamiliar/strange locales (underwater, aerial, magical environments, etc.). Combat while infected with a disease, under the effects of a curse. Mis-information gleaned about an opponent can make a fight harder and so on...
And yeah I'm sorry but the terrain is part of the exploration pillar just as much, if not more that combat, I find it strange that you regard terrain as a combat effect. Terrain literally determines the available modes of movement along with how fast you can move outside of combat.
Is it really strange? What does traveling across sand do compared to traveling across a thick brush? Both are difficult terrain, both are simply doubling the time it takes to get where you are going. Even if you do something that deals damage to the party while they walk, if they can survive it, it doesn't matter unless they get in combat.
Some of the other things don't matter at all. Cold and High Altitude would have no impact on combat at all. Meanwhile things like rain ONLY matter for combat. Meanwhile the disease or curse, well depends on what they are. Certainly a curse that causes you to take extra damage makes combat harder, but if I added that into a combat... I'd just do it as hex or Bestow Curse. But is an environmental factor exploration? I don't think it is.
Orrrr... It's a climable surface whose DC is set by the DM, not the DMG and as such I can say it's an easy climb during the day if you can clearly see or know the path of handholds... a moderate climb at night under full moonlight and you can clearly see or know the path of handholds... a hard climb if you can't see the marked hand holds and only have moonlight to climb by and very hard if you are climbing totally blind on a moonless night.
In this way darkvision could be an asset if you don't use torches or lanterns and are climbing under moonlight (perhaps you can see gradations of gray where the handholds are marked... it's up to the individual DM) but it doesn't trivialize or auto win this challenge.
Sure, you can make up DCs, but then you are simply ignoring the guidelines in the book, which means those guidelines aren't actually doing anyone any good.
It does work with baseline rules... optional rules can be used to enhance it as my example showed.
Right, but we aren't asking about enhancing the situation, we are trying to get into the baseline situation.
Yep because it's totally not a trope in adventure fantasy... not at all...
Of course it is a trope in fantasy. That doesn't mean it is a good trope, or that I'm not going to complain when we literally tied the child to their bedroll and had someone on watch for just this occasion, but the child slips free and wanders off anyways.
One issue with tropes is that while the DM may want to implement, the players know them too, and many tropes irritate people who realize that rubbing two brain cells together solves the issue.
A high level party should be exploring the planes... and yes that can be a challenge for even them.
Is it? Have you looked at the rules for the Planes in the DMG?
The plane of Fire uses the Extreme Heat rules, so no penalty if you have water. They have strong winds that have no effect outside of combat, and thick ash. Probably counts as precipitation since it says the air is breathable. So, carry water and get some face wraps for the ash, and you are perfectly safe to travel the plane of Fire.
Sure, you can add things to this, but per the written rules in the DMG, that Plane of Fire is no more dangerous than a desert. And you have to be careful adding things, because if you say that (for example) there is a field of burning embers they have to cross, taking 1d4 damage per round... if you need them to travel for three hours they are dead. That is 600d4 damage per hour, or an average of 4,500 fire damage total. The only way to survive that trek is Fire Immunity... and as soon as they have it, the trek is no challenge. Because the only challenge was surviving the damage. It is why damaging terrains are so rarely useful as a mechanic.
But, let's look to a different plane, what about Hades? It has one of the nastier effects in the planes right? The only rule for Hades is that you make a DC 10 wisdom check vs Exhaustion at the end of every day. If you get six levels of exhaustion you are permanently turned into a worm creature.
Is this really a challenge? I mean, sure, exhaustion after every sleep is bad, but we are talking high level players. They have access to Heroes Feast for advantage, they have access to Mordenkainen's Mansion to sleep in a pocket dimension instead of the plane of Hades. They might be able to just make the rolls, a 15th level cleric is likely making the save at +10... meaning they literally cannot fail.
And if things get too close to bad? Well, if they got to the plane via PLane Shift... then they can leave whenever they want, before it even gets close to this Larva transformation. So again, any real challenges beyond a single easy saving throw at night have to be manufactured by the DM, there are no guidelines for any other challenges in exploring the planes, and most of them are this way. Very easy DC saves, once a day, sometimes for nothing but an alignment shift, which is aggravating for entirely different reasons.
