While it is by no means finished in terms of its applications and features, the new C screen for showing a map overhead of the entire world (with remarkable clarity and no fog of war mind) is very impressive. Featured within the slew of hill dwarf features i feel like are arriving at a relatively steady & stable pace (albeit not perfect, getting there) the particular snippets of the Hill dwarf arc are a little bit too tantalizing to pass up talking about figuratively in how they could turn out.
The subterranean map layer
From legends mode, the subterranean is largely unexplored and un-named zone placed beneath the over-world, the dwarves knowledge of it stretches back into a archaic and hostile past of being naked, afraid & prey for jabberers and minor incursions into the surrounding darkness of the first cavern layer still yet lay the true knowledge of what (without cheating and peeking behind the veil of RAW's its much the same to new players) lies beneath. Prologue aside, the suggestion is split into two halves of how a C-screen could be implemented.
Exploring - The subterranean works differently to the overworld because initially there is no concept of destined travel unless its between two areas your civilisation is already aware of, location can be correlated but each section of cavern (which will have to be redesigned mapwise) needs to be 'discovered' by a expedition team of dwarves, the first civilisation to pass through a cavern area get the priviledge to name it, and caverns might already be named according to the residents and of course populated by its own hazards like roaming forgotten beasts & wildlife.
> If there is a purpose to going below the first layer, as to hunt for location ruins (*cough* dark forboding fortresses & vaults *cough*) then the ability to flip terrain planes and undergo a higher level of difficulty encroaching on forboding and undiscovered second and third level territory is possible
> [SCOUT] nations with allowed mining jobs and subterranean access via sites will be able to explore the first layer themselves, goblins already establish underground tunnels that allow them to move across distances of oceans and mountains, meaning a aggrieved goblin civilisation on a isolated island may eventually find you with enough scouting naturally and make hostile "first contact"
Meeting and beating the Dwarfbucks out of locals - Kobolds, Animal Men Groups & Deep dwarves inhabit the lower layers have very little to no contact with the above-ground world, and while they are primitive they do often have their uses (besides kobolds being used as theiving loot pinata's, underground kobold sites are not hidden but still need to be explored) as raiding targets and more easily (?) dominated tributaries.
Making first contact & travelling - Not everything has to be malignantly intentioned, a good introduction to the primitive peoples & the Deep Dwarves may be rewarded by amicable mutual trading opportunities and visitors from the depths (not of the hopefully !!fun!! kind)
> As previously mentioned, hidden areas of the map can be revealed and settlements virtually all over the overworld can be reached through slightly ardous and dangerous walks through the subterranean layer. Expeditions may lead to treasure and riches or to certain doom by opening up a cavern you most certainly should not have opened with exotic and rare dangers.
This was only a short suggestion (until i come back to it with fresh material) but i hope you enjoyed reading it, it would be a nice insert to the 'exploration' ability we have now that is lacking interactions with the above ground world (if i wanted to raid a human tomb for instance) and the old pre-tavern arc landmark features that add a little bit of world-generation flavor.