4Game

I love the new designs the newer dungeons have in the recent updates. The older dungeons could have an overhaul to be at the same level as the Mountains and Mistlands dungeons in the future.
This can be achieved even by only using existing assets.

Black Forest dungeons are currently very simple dirt and stone corridors (which is fine as they are the lowest tier).
The layout could be expanded with wider and/or taller corridors of dirt with the Elder roots from the boss fight spawning randomly from vines, and candles decorating some tombs.
Imagine the Deku tree from Zelda OT and nordic dungeons from Skyrim but minimalistic and simple, in Valheim style.

The Swamp crypts gameplay consists on: "walk 1 meter, break the iron and goo ore, walk 1 meter, break the goo and iron ore.." which makes them boring and not enjoyable. If some more rooms like medium sized prisons with more draugr on them and maybe different floors were added, it would break that monotony of "get iron and bonemass location, never come back".

There are no dungeons on the Plains.
But an idea could be simple black metal mines with miner fuglings on the interior wielding pickaxes. Here you could farm some ore and coins.

All dungeon tiers may need a new item to loot like a cosmetic or a new piece of furniture (a trophy?) like a decorative prop/structure resembling the dungeon aesthetic.