Before we explore the premise, let's consider development limitations
As getting to the Shattered Planet is something most players will never achieve (and that's by design), it make sense that the devs in this hypothetical would avoid making engine-changing mechanics just for the Shattered Planet, and in general would want to keep the scope small.So, while it would be cool to move chunks of the map around with space thrusters, as suggested in this thread, I doubt this can be created within the existing engine constraints. Same can be said about having more than 1 Landing Pad per surface; even if you could implement that, it'd be an unneeded inconsistency with the rest of the game.
There's a Shattered Planet mod that I deliberately did NOT look into while writing this post, so that my thinking wouldn't be influenced by it. I might end up writing a follow-up comment after playing the mod.
Gimmick: Limited space
I imagine the Shattered Planet as "islands" of landmasses floating in Space. Kinda like Fulgora, but with empty space instead of oil ocean.While limited space is your constant enemy on all planets except Nauvis, you eventually unlock means to just say "nuh-uh", and even until then, the space you have is sufficiently big for non-megabase purposes.
So, for the Shattered Planet, the "islands" should be pretty small, and there should be no* way to bridge the gap between them or expand the islands. You work with what you've got.
Since this is a post-endgame location, using high-Quality Space Age machinery could be an intended way of playing the planet is balanced around, as opposed to standard-quality Assemblers and Chemical Plants.
While proof-reading this, I thought of a Shattered Planet unlock that'd be useful primarily on the Shattered Planet, but would have a use in the rest of the game - Guaranteed Quality Upgraders. It's plausible that the player would've landed on the Shattered Planet without Quality Modules whatsoever, and bootstrapping both the new base and Quality Quality Modules for the second time, from scratch, and without support from other planets wouldn't be fun.
By the time you reach the Shattered Planet, you've probably went through the Quality quest, so, Guaranteed Quality Upgraders would be a QoL and anti-screwup addition that wouldn't obsolete your existing setup.
Resource transportation
Thanks to the Mecha Armor, you have no problem moving between the islands yourself, but you still need to move resources, or at least the end products (whatever they may be), between islands.And here we encounter a game design dilemma. The idea of each island being its own independent base is appealing, but some players might find this a dealbreaker - a limitation too anti-fun. We need a solution to move resources between islands that is acceptable if you REALLY want that, but one that's disincentivized and "feels wrong".
I don't think changing the way rocket logistics work just for the Shattered Planet "surface" is an elegant solution, so, let's keep 1 Landing Pad per surface.
My solution is Roboports - world generation should ensure that you can bridge the gap between islands using Roboports. It doesn't have to be two large landmasses being that close together - simply having tiny islands ("asteroids" I guess) close enough for the Roboports to connect and daisy-chain between proper islands should be enough.
I've happened to "playtest" this idea on Vulcanus in a peaceful playthrough - while peaceful Demolishers don't intentionally target your buildings, they still break anything that happened to be in their way, including your belts. One way around it is to have an isolated logistic network that simply moves the tungsten ore across the "gap" created by the Demolisher's patrol path. (until you have green underground belts which are long enough to span the gap)
Also, the Shattered Planet could use Rocket Silos to ship resources onto a space platform and back down to the central hub area.
Resource production
For basic resources, we could continue the space/asteroid theme and have you use Asteroid Collectors to collect chunks, and various turrets for breaking down larger asteroids. Introducing new types of asteroids would be underwhelming; materials such as uranium would be more interesting to obtain in a different way that's neither asteroids nor ore patches. Although with uranium specifically we can simply not add it there, as it's not a requirement for a fully capable base.At first, I've considered an idea of meteor showers being an environmental hazard akin to Fulgora's lightnings, which would require you to build turrets throughout your base. This would be both a defense mechanism and a way to gather resources. However, I have a hunch the idea is inherently anti-fun, would be hard to visually convey, would waste precious land space, could set you back on a planet that's already quite brutal, and would require the devs to make a whole new gameplay system.
Because the journey to the Shattered Planet is a hard one, the planet surface should be self-sufficient and not have the hazards that could set you back. For example, I imagine that free-floating asteroids could damage buildings on the edge of an island (primarily asteroid collectors), but they couldn't damage things on the interior.
For planet-exclusive resources, we run into another game design conundrum. They are supposed to be planet-exclusive, and having alternative recipes for, say, most of the organic Gleba stuff, would feel wrong. Having ways to produce raw planet-exclusive materials like tungsten and bioflux also feels wrong.
Being able to clone or synthesize a resource via, say, Promethium, would also be a bad idea; Minecraft has a mod called Equivalent Exchange that explores the idea of transmutation, and it practically becomes creative mode once you've got enough of any single resource that you can endlessly feed into the transmutator. To find an answer, let's discuss...
Progression
Do we want Research to be conducted on the Shattered Planet's "surface"? Giving that the Shattered Planet is so far away from everything else, the only "product" with reasonable delivery time is Research - done by creating and consuming all Science Packs on-site. The problem is that while an alternative recipe for Stack Inserters is not a big deal, same can't be said about Agricultural Science Packs.To answer that question, we need to remember that you need to travel in-person to any planetary surface first. In a normal playthrough (we'll discuss megabases later), the player is likely NOT going to travel back, and the Shattered Planet is their final destination.
With that in mind, Science could be relegated to the familiar planets left behind, while you build your Final Base on the Shattered Planet. This separation of duties would keep the other planets still relevant after reaching the Shattered Planet.
With Science Packs not being required on the Shattered Planet, alternative recipes for Stack Inserters, green Turbo Belts and some other things wouldn't feel that wrong, especially since alternative recipes (e.g. Rocket Fuel from Jelly or Ammonia) are already a thing.
Having to research these alternative recipes with Promethium would also give it an actual use. Promethium would be required to both research (which requires collecting Promethium at the System Edge) and craft the items (with Promethium obtained on the Shattered Planet).
Purpose and end goal
To generalise, there's 2 types of playthroughs: just beating the game or building a megabase. (For our purposes, speedrunning counts as beating the game)For merely beating the game, conquering the final challenge doesn't have a use besides it being a win condition. This applies to both the Base Game (launching the rocket is a win condition after which you abandon the save file) and Space Age (getting to the System Edge).
If you could build a new base on the Shattered Planet's surface, it'd become a new "true ending" for players with dedication.
Since there's no achievement for reaching the Shattered Planet, it makes me assume this task is considered too brutal even for achievement hunters. If that's the case, an achievement about doing something there would have the same problem. Or maybe because it'd make 100% the game unreasonably long.
Either way, having this new planet to explore and build a new type of factory at would enticing for people to get there, which would require proper bases on all planets, including Aquilo, while currently you can kinda sorta get away with basic cobbled together setups.
The value of the Space Age DLC is that it moves the finish line much further away, and you get the time to play with [what used to be] end-game unlocks like beacons, modules, nuclear power, etc in a non-megabase playthrough. Having the Shattered Planet as a planetary surface would move the [optional] finish line even further away.
While yes, you could have "reaching the Shattered Planet" as your personal win condition, it feels hollow because while the effort is immense, there's literally nothing at the end of the journey. The opportunity to build a new base on a new planet, even if not as content-rich as others, would be something.
When it comes to megabases, a multi-hour travel to and from the Shattered Planet is not that long. It's important, because the items crafted exclusively on the Shattered Planet (whatever they might be) will arrive in infrequent, but massive packages.
As I haven't played a megabase playthrough, I don't know what new item would be beneficial for megabases, but wouldn't obsolete the designs that don't use it, making it feel like you have to make a Shattered Planet surface setup before doing a megabase.
My first reaction was to add Quality of Life improvements that don't necessarily make production faster - Electric Trains or multiple Landing Pads per surface, but it's such a tease. "Want this thing that'd be very convenient in a normal playthrough? Then beat the post-endgame content so brutal it doesn't even have associated achievements!" So, we'd need to avoid unlocks that'd be useful for non-megabase applications.
Development feasibility
On a purely technical level, I believe what I've outlined in this post is a reasonable ask even at the sunset of Factorio's development cycle.What may or may not be out of scope is balancing and playtesting.
From an outside perspective, it should be reasonable - the Shattered Planet Surface being a post-endgame content seen by the few and mostly relying on already existing content makes balance issues less likely, and not that impactful. Also, it's reasonable from the players to expect that what's basically a "secret level" would have less effort put into it than the rest of the game.
I believe the devs have a good hang on how to balance things to not require major overhauls to the Shattered Planet after its release. At worst, any plausible Shattered Planet balance issue will only affect the Shattered Planet itself.
However, people tend to underestimate the effort that goes into game design, and I am a people.