And yes, expanding on the economics would be nice. You could add two new axis to represent differing ideas on economics. Individual vs Collective ownership and Market vs Command economics. (Collective markets would basically be companies being owned by their workers while Individualist Command economies would be more akin to either full on feudalism or Megacorporate societies; either way the economy is essentially in the hands of a relatively small number of people). Adding those axis would definitely need more ethics points though. At least five if not seven. From there you could add civics to further represent all the smaller nuances inside like democratically planned economies or corporatist systems or whatever.
Trading would need more goods in order for trade to be anything besides an additional source of minerals, food, and energy credits though, which mind you is entirely workable since that's how a lot of 4X games have done it since time immemorial, it'd just be kind of disappointingly basic. Not just strategic materials but things like finished products. From paradox's history there's basically four different approaches, there's Victoria's approach which is extremely in depth and can be kind of confusing (though amusingly because of things like money spent disappearing forever instead of being distributed to the POPs and some bugs in the inflation system the economic system will invariably break in hilarious fashion) and more towards the simple end of the scale you have CK2's system which leans more towards trade just being another source of income/causus bellis.
Hearts of Iron used to be by far the simplest system since well, Hearts of Iron is laser focused on simulating mid-20th century total warfare so most of the depth is going into the military production system and combat, before HOI IV expanded more on the idea of strategic resources and trading factory output for resources you need for other things. EU has something of a balance with trade goods and some rather basic markets and monopoly systems for them.
While I'm sure there are many of us who would enjoy Stellaris with Victoria level depth to the economics I don't think that those kinds of systems are good to introduce to games not built around them from the ground up. Something closer to EU's system with bits of Hearts of Iron's and some ideas taken from Victoria would probably be the best.
Now of course when it comes to trading there's the question of whether trade ships get shown on the map or not. Traditionally paradox has always erred towards not making merchant traffic a physical thing on the map in any of its games to cut down on the lag that a massive amount of civilian traffic would produce. Instead having routes and sometimes making merchant marine ships things you have to build some; being a resource that's interdictable and depletable by ships squatting on the trade routes while still not making them selectable/conventionally targetable units like warships are. I'd think that the way HOI does it would probably be the best example to look towards myself. Maybe giving those merchant ships little models on the map but not making them units the way warships are. At most I'd only support special trade mission ships who aren't for daily civilian traffic but as the name suggests; special trade missions.
To add onto the idea of a shipyard mega-structure, I could also go for a special trading port megastructure that serves as a hub for trade, along with smaller non-megastructure trade posts. Though generally trade posts exist because they're in a convenient stopping point for traders. And you would need a reason to stop at them instead of just continuing onto the planet directly since for everyone not using hyperdrives there's no such thing as geography in space. I'd imagine regular trade ports might only be buildable on "nodes" visible on say, a trade route map mode. Meanwhile a megastructure trade port is not only a node in and of itself but it might also do things like speed up FTL travel to it; shorter wormhole opening times to the system, warp travel goes faster, jump drives spin up faster, and it creates new hyperlane routes towards itself.
It'd probably also create quite a lot of energy credits obviously. While a star forge mega-structure would create both a lot of minerals as well as create ships quickly, and quite possibly make ships too large for standard starports to handle such as dreadnoughts and titans.
Of course some ships (if what you want to make is more comparable to the Deathstar or a Blackstone Fortress in scale) would necessarily have to be too large for even Star Forges to produce and be mega-projects in and of themselves as any ship capable of basically single handedly sterilizing/obliterating a planet/star system should be. I'd imagine that creating any such super weapons would probably attract a lot of negative attention as well as inspire both rebels against you and foreign enemies to try the construction site of a weapon before its ready to unleash terror upon the galaxy. And like the death star, the superweapon should probably have some limited functionality to defend itself even if not 100% complete (I'd imagine any death star knock offs would have to be built in stages), much like how the Second Death Star could engage the Rebel fleet despite only being two thirds or so complete.
A combat capable artificial planetoid should probably be at least somewhat customizable so you can decide just how it brings the doom of your enemies ranging from "just sterilize the planet" to "obliterate the planet" to "blow up the entire solar system" to more esoteric things like fleet/infrastructure EMPs, matter forges to suck up matter from fleets, stars, and planets (or in simpler terms; just pulling a Unicron and literally
eating the whole planet) to fuel systems that can quickly produce fleets and armies, or Yuri's revenge esque mind control systems to enslave whole systems, how much defensive/secondary armament it has (and whether it's got a ton of strike craft or not) and whether it has/what kind of secondary abilities it brings to the table like souped up versions of the auras that defense stations bring to the table.
Some smaller scale superweapons like massive gun stations that can fire across interstellar distances would be pretty cool (something on the level of the Star Wars EU'S galaxy gun or the film canon's star-killer base that can delete whole planets/star systems from across the galaxy would probably be absurdly broken); inflicting damage on enemy ships within their range as long as they're spotted.
Obviously, the event-pop up image for first testing out your superweapon should feature a spherical superweapon bringing about an artificial lunar eclipse on an unlucky planet and firing a beam. Because that shot from Rogue One is simply too cool to not homage. And one of the responses for the pop up for when another empire first destroys an entire inhabited planet/megastructure or star system with said super weapon probably should feature the quote "it's as if millions of voices cried out in terror and were suddenly silenced". While for the user at least one of the responses for the event pop up you get when you first use it to commit mass murder should be a Tarkin quote of some kind, though other quotable options include the Exterminatus speeches in
Retribution or from
Battlefleet Gothic. This is also
an acceptable source for 40k quotes regarding killing worlds.
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