I really like a lot of the concepts you're playing with here, many of these megastructures fill niches that aren't covered in vanilla or Gigastructural Engineering, and some are VERY creative. I was especially fond of the Space Museum and Orbital Gardens! I would like to offer some feedback and suggestions in regards to some of the megastructures though, both to answer your soliciting ideas in the description and for balance in general.

-Astro forge jumps from 220 to 3300 alloy at final upgrade; would suggest ~500 instead. 100% empire wide boost is drastically too strong, 30% or so would be better.

-Astro Accounting doesn't work at all currently; for when it gets fixed, the listed values in file probably gives too much admin cap, but I'm not sure what a fair number would be, maybe try 50%?

-Black Hole Siphon gives a useful boost and is already solid. Maybe add a % bonus to dark matter production, around 30%? A flat mineral boost of ~500-700? Or a boost to Physics research could work.

-Gene Bank is unbelievably overpowered with that 100% job modifier. 20-25% would probably be fine alongside the other boosts.

-Magic school is reasonably balanced. Maybe add a 20-30% boost to unity production?

-Nano Forge not actually buildable ingame; for when it's fixed though, it probably gives too little of the nanites/living metal. This could be a good place for a relatively small (maybe 10%) megastructure build cost reduction or a 50% megastructure build speed increase.

-Nursery is absolutely insane with that 100% pop growth AND happiness, and there's already happiness boosters. I would drop it to 30-50% pop growth.

-Orbital Garden jumps up a bit too much food imo, would rather see something like a 250/500/750/1000 spread. Also, at the final stage it adds 0.2 food instead of whatI assume was supposed to be 20% food production? Otherwise I actually really like it, the additional bonuses are very well done!

-Space Robots gives WAAAAAAAY too large of a job bonus. 25% to go with the upkeep reduction would probably be better.

-Space Force One giving 40 stability and -40% crime empirewide is crazy strong, would suggest 20 each instead

-Microverse Maids are a hilariously awesome concept, but feel a bit too strong, maybe reduce to a 40-50% amenities boost? It's not far off though, and the concept is great.

-Space Refineries is great as is, I like it!

-Spacey's Mall gives far too little base CG, but drastically too strong empire mods; 500 CG and ~30% consumer goods mult, dropping the amenity boost entirely since there are dedicated sources for that, would make it very solid imo.

-Seed could maybe just give +X% universal habitability (pop_environment_tolerance) rather than a modifier for each type? I like the idea but it's really hard to read atm.

-Intergalactic Expo is a great concept, 15 minor artifacts is a cool and unique bonus. Since expos tend to be about meeting people and demonstrating products, what about an empire wide trade bonus? Maybe 30% or so? Or 30% empire energy income instead, so hives/machines get use from it.

-University is a cool concept and I like the mix of boosts, but universal job bonus is pretty crazy strong. I think dropping it to 20% would be better balanced.

-Space Vault is WAY too good, making all starbase building literally instant is just too much on top of the megastructure bonuses. Honestly in order to balance it, I'd suggest completely removing the build time reduction and halve the rest of the bonuses. Hell, it'd still be insanely good even if all it gave was +2 megastructure build cap.

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Overall, the infinitely stacking natures of these is insanely powerful, get enough Space Vaults and you can pretty close to instabuild any megastructure for example. I'm not sure what the solution is, but I figured I'd bring it up anyways.

I also noticed fairly severe placement issues, often times despite having them build at different planets I'd end up with multiple megastructures stacked on top of each other which can get very difficult to select.

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I hope I didn't come off as rude in this post, I really like the ideas, but having the megastructures be more balanced would make this a great counterpart to Gigastructural Engineering in my opinion :)