I think it's been established at this point on various occasions that both jorb and loftar like the idea of expanding the depth of the underground but wanted to also expand the content of underground as well.
So here is my input -
Basically it's a pretty popular concept in many games with mining, from your average android digging game to Stardew Valley to Terraria. Mining levels come with themes attached to them. As you go deeper you pass through the "ice layers" or the "fire layers." This is what we need for endgame.
This idea would keep level 1-5 as they are and effectively make them the "standard, non-endgame" version of mining. If mines are fleshed out with more base level curio and forageables, maybe some small critters and whatnot I think that levels 1 through 5 could basically be themed as perfectly generic and stay just as they should be.
Levels after 5 could come in additional groups of 5 and be thematic. Perhaps going passed level 5 would need a very late game credo. Maybe after Cave Hermit, Strider and Scholar and Pearl Diver you unlock the Spelunker credo, which unlocks access to a new machine construction that can help you make the special level 5+ mineholes. I am thinking that after getting down to level 5 (especially if they add some difficulty to this like adding hardness restrictions to making mineholes), you should have some fair mining industry. So maybe in order to advance beyond level 5 you should have to construct a siege-like construction called a Bedrock Drill. Made from various glues and steels and woods and all that good stuff. Maybe you can sort of prospect for a weak spot in the bedrock floor of level 5 and construct this big drill in pieces and run it like you would run a wrecking ball or ram. Just have 1 or multiple chars on it with high strength stats, or even hook bulls up to it and have them turn a massive drill for many hours or days to create a hole that you can use to make a minehole construction on afterwards.
After that the expansion can be endless and left for future development since the requirements to actually progress this far should take some real time. This could easily make way for various mystical metals and gems at lower levels. Think orichalcum and mythril and adamantine. Also, way more insanely difficult mobs. Themed slimes and trolls and worms. Deadly terrain and atmosphere. Various new liquids, plants, cheese maturing environments.
Maybe level 6-10 is not fire or ice but rather like junglesque mesozoic theme with dinosaur bones to mine out, oil, petrified artifacts and crazy dinosaur mobs and ancient plants that can only be farmed in this environment.
SO MANY POSSIBILITIES