Suggestion - Production Anomalies
I'd like to see a most robust, engaging economy. There's a lot of ways that this could be done, but here's a couple initial suggestions, and a couple long-term suggestion that could be implemented only after an economy overhaul.
Take the research anomaly system that already exists and expand it to include anomalies that can boost or alter (or nerf!) production of specific items, whether it's specific trade goods, component categories (lasers, missiles, shields, cloaks, etc), or installations (mines, factories, PDCs, infrastructure, etc), or even just a direct boost to 'wealth' production. This change alone would be enough to tempt players to spread out their production. It's a big change from the current situation where, more often than not, the homeworld (and sometimes planets that have been terraformed/grown enough to become essentially identical to the homeworld) is where all meaningful production takes place and all goods go to and from it. Instead, you'd have the option to boost production by having worlds dedicated to certain products, from which goods would then have to be shipped where they are needed.
I'd make these sorts of production anomalies rather frequent, perhaps with more rare types that have more dramatic effects. The idea is to make colonized worlds more unique, especially in terms of economy, and to give the players a choice between more efficient but spread out production (more/better goods at the cost of the delays and vulnerabilities that come with shipping,) or more concentrated production (which also allows for more concentrated defenses) but without making full use of production anomalies.
Alongside this I'd also recommend re-looking at the Trade Goods system, where the goal would be to make them more meaningful and impactful, and especially more engaging for players. One possibility that I'd considered was giving some planets the ability to produce unique/rare trade goods that have special effects, such as increased population growth or better environmental tolerances to populations that have access to such a product. With it being planet specific, again, trade between worlds is made more engaging and meaningful, and the choice of which worlds to develop and how become more difficult and impactful.
With this, I'd include a way for players to choose between more interventionist trade policies (government gets more or less say in where trade goods are shipped and how many of each are produced, as well as setting import/export taxes and tariffs on specific goods, like a tea tax) or more laissez-faire trade policies which would be the current system where trade goods are solely the province of private citizens/organizations (shipping companies and the colonists that produce the goods determine what happens with them, with little to no input from government/player.) Trade policies can affect production growth rates, maximum production capacity (probably per colonist working to produce the goods?), or a host of other possible effects to choose from. They could and probably should come in both domestic and foreign variants, where the domestic policies control how much of which goods are produced, where, and how much of what can/should be shipped to which planets, and foreign policies regulate how much of which goods we are willing to trade to and/or from any other specific empire (a full ban on Cuban cigars so our citizens can pretend they're somehow better than other cigars by virtue of being harder to obtain!) Policies regarding immigration/emigration between empires could even be introduced alongside (or after) this is implemented, using a very similar system.
Once trade goods are improved, we can then even start looking at more trade-related diplomatic options (trade sanctions against specific products, for instance, to allow trade between two empires while still denying access to unfavorable trades/markets.) There could even be things like trade wars, where total warfare is not permitted (no land-invasions, possibly other restrictions) but enemy shipping/fleets are free game, and planetary/system blockades could be a main feature of this sort of limited engagement. This is only as meaningful/impactful as the trade goods in question, though, so making those more important would have to come first.
Legality of goods is another thing to consider for after trade goods have been overhauled, where we could include things like military grade stimulants which you may or may not want in your military (short term combat effectiveness boosts at the expense of nasty side-effects?) but you certainly would not want in your civilian populations (increased Unrest relative to how much of the product is available in your population?) Could also include things like slave labor, legal in empires which practice slavery but illegal in empires where slavery has been abolished. There's a whole host of products that could play nicely in a system like this. Legal (read: minimal diplomatic effect) destruction/seizure of ships belonging to other empires when they are caught carrying illegal goods through your space could be a thing in this, along with a cargo-scanner module and perhaps other stuff for 'military police' ships (or a Coast Guard, or whatever you want to call your empire's Customs officials.) Alongside it, piracy and smuggling could be introduced.
But really, the suggestion is just to keep economic gameplay in mind when re-coding everything in C# so that at the very least it can be easily expanded on once economy becomes more of a development priority. Personally, expanded/improved economic gameplay is a top request of mine, so the sooner this happens the better. (Okay, maybe I just want more fun and impactful space-submarine and trade-interdiction gameplay, so sue me!)