New world type: Apocalypse

The overworld is the same except Lava and water swap positions.

This would be challenging to gathet resources as exploration is usually a fantastic way of finding villages, pllager outposts etc. This will become a lot more difficult in this world type. Forest fires will become more common

Water will become scarce so farming will be difficult to start with and ice will become almost as rare as emerald ore and only appearing on Y levels greater than 90 (easy), 95 (medium) and 100 (hard).

Different difficulties could have a different lava level. Easy = 64. Med = 72. Hard = 80. The harder difficulties could mean that deserts are completely submerged in lava so cactus is going to be nigh impossible to come across. Also, magma cubes and zombie pigmen can spawn in the overworld (it is called apocalypse).

Now, of course I haven't mentioned the elephant in the room. Ocean monuments and ship wreaks and other underwater structures. I propose that generation of those structures stay the same, however they are encased in an obsidian sphere. Conduits will become obsolete so shipwrecks not being a thing in this world type make sense. If you want to keep conduits/Heart of the seas you could add them to the potential inventory of spawners.

And if that isn't challenging enough, one word... AMPLIFIED.

This may end up being a throwaway idea but it would challenge the minecraft veterans.