Better Progression You Can Easily Implement.

Easy progression to program:

(Because you decided to use voxel worlds in static locations, that means you can have a Kerbal Space Program style of progression on an easier level.)

-Firstly, you can add science blocks that when taken to a specific location at x,y,z and activated it will Science Points/Research Points or whatever you want to call it.

(Locations can be lakes, valleys, mountains, and the multiple biomes you have. It can also be bonuses for exploring new planets/moons)

-Secondly, you could insert resources (stone/magnesium/iron ores) into science blocks in order to research new smelting ingots, then research ingots into components, and then new components into new blocks.

-Thirdly, you can make a tech tree menu like the progression menu already made in game players can pick and choose to go down. (If you want more information about what I have in mind on, I can go further in-depth)

Hard to program:

(Take inspiration from Civilization/Stellaris games and focus on rewarding city building and faction making. Where you claim territory and go to war with other factions. Because its a fair bit more complex, I'll only add a skimmed version of what I have in mind.)

-Ideology,

allowing you to make religions for bonuses with its own goals like converting every planet.

-Society,

having laws, owning land, controlling entire planets, forming governments. Getting rewards along the way with set goals.

-Proper Enemies and Enemy Factions

(This is probably the hardest to make because this is a physics base game, and making AI that's not trash and crash is a challenge when working with vehicles and ships. But its entirely necessary because without it players will have no real reason to go to war, unless you give them reasons to fight with each other hence the land owning.)

The things that are missing from Space Engineers that will make it soar in popularity are: Progression, Exploration, and a Threat. I hope you consider what I have formed in this and will consider to work on making Space Engineers better so it will grow, because it has so much potential! And if you have any questions or want any more feedback feel free to either reply or email me.