To the Dev team, love the game and graphic style. Although, while you keep busy in the beginning, the game quickly plateaus. Still 10 out of 10 though.
For the purpose of this post, the term outpost will be used to represent ones 'base'.
Cosmetic/Decorative items and buildings
Ease of play objects
Intermediate end goal
Game flow improvements
Cosmetic and Decor
Walls/Fences: We all love the terrain tool, but for the more aesthetic oriented players it can be quite painful to attempt to create something pleasing.
Snap-in Structures/Add-ons: Creating your outpost is a large portion of the game, which can easily become a chore if you enjoy things fitting perfectly. Creation of snap points at/near the corners/cable plugs of platforms(in 45 degree increments) could make organization and layout a breeze. Also consider a choice of shelter shape when beginning a new world, either square or traditional circular.
Potted Flora: Who doesn't love a nice houseplant? Creation of a 'planter'(various sizes could be added as well). All defensive/offensive flora would be scaled down to a proper proportion depending on the size of the planter, and would no longer attack or harm the player. An additional use of the planters could be harnessing the bushes that regrow research samples as a way of 'farming' research.
Colored Lighting: Fairly self explanatory.
Structure coloring: The ability to color or paint structures -OR- have the character's skin colors(such as the Galactic colors) applied to the building when it is crafted, within a reasonable extent.
Complex: Advanced version of a shelter, with the ability to place items/objects in such as planters/Geometric Triptychs. Possibility of additional rooms for your fellow Astroneers, could be a built in function or act like a modular home, with separate rooms being added as modules snapped onto open airlock spaces.
Examples of potential room modules could be: Personal Quarters, Greenhouse, Planetarium(Triptychs would be placed in here and activate a scale model of the Gateway Portal), Generator Room(RTGs or any other type of self sufficient generators could be placed here on one large placement node, up to 2 RTGs). The interior of the Complex would be a load barrier, and would take that player out of the world, to reduce the amount of players active in the world at one given time.
Ease of Play
Advanced terrain editing: A tool or large rover attachment that can flatten an area within a certain radius. With mod slots similar to the terrain tool. The mods would essentially function as they would on your character. Wide mod could increase the radius, up to maybe 1.5x or 2x standard character reach with (x) amount of wide mods attached. Boost mod would speed the process. A filled analyzer mod would flatten and paint the new area it has created. Soil canisters would be required, and the energy requires would be large.
Tunnel bore vehicle: The rovers while using a drill are incredible useful, but the major downfall(almost literally) is that the planets are basically made of Swiss Cheese. A tunnel bore could solve this issue by not only clearing the area in front of it, but also laying new ground as it goes, preventing any surprise pitfalls, while also creating a convenient tunnel. The shortcoming of this could be that any encountered resource is lost, as the tunnel bore would only have (x) amount of slots restricted to terrain tool mods and soil canisters.
Paver: Similar to the tunnel bore, the 'paver' would alter the ground in front of it to make a road or path. With the material required to lay the road being created at the Chemistry Lab. A potential recipe for the 'concrete' could be ceramic+full soil canister in the solid slots, and a canister of water in the gas slot. With a moisture condenser being a new structure or having water added to the Atmospheric Condenser.
Intermediate End Goal
I labeled this section 'intermediate' because I am unsure of your goals with the gateways and leave that reserved as the true end goal.
Five levels of power consumption: [1=Self Sustaining] [2=Sylva Chamber equivalent] [3=Novus Chamber equivalent] [4=50U/s] [5=100U/s] For comparison, a single RTG produces 4 units per second(4U/s).
Terra-formation: An obvious route for a space sandbox, this option could create a world of content and new structures. This section is going to be quite large so bear with me.
Extra Large Printer: Allows for 5 slot crafting, used to create the Massive Printer, EM Field Generators, and Nutrient Processor.
Massive Printer: Allows for 10 slot crafting, used for Atmospheric Filter, Cloud Seeder, and the Terra Engine and other items.
Atmospheric Filter: Used for the alteration of a planets atmosphere. Power use: 4
EM Field Projector: Used to alter the electromagnetic field of the planet. Could snap on to 6 axis points of the core. Power use: 1
Cloud Seeder: Used to alter/create weather patterns to distribute nutrients across the planet, this structure is part one of two and will require an addition building placed adjacently. Power use: 3
Nutrient Processor: Used to create the necessary nutrients for flora and potential animal life. Power use: 2
Terra Engine: Used as the final step in making out-of-suit life possible. When all stages above have completed, the engine will notify the player in some way that the planet is sufficiently ready. Once initiated, the player will be prompted to leave the planet before the process begins. The planet will be unavailable for travel for (x) amount of in-game time. This process will remove all structures, items and objects. The geography of the planet will change, and instead of specific landing zones, you will be able to land anywhere. Power use: 5
Tritium: Used in the EM Field Projector as a self sufficient power source. Additionally could be used as a permanent light source.
Magnetite: Also used in the EM Field Projector, along with enabling new means of power generation.
Astronium utilization: After researching (x) pieces of Astronium, you discover it has some extremely unique properties. These properties could enable 'Resource Compression' allowing you to have a type of box dedicated to a single type of resource that only takes up one backpack slot. Each box can hold up to (x) amount of that item and would reduce the amount of entities rendered in at one given time to help reduce late game item lag.
Cold Fusion Reactor(CFR): An end game level generator that produces (x)U/s and takes up a large placement node. Could be placed inside of the 'Generator Room' Complex Module(Refer to Complex under Cosmetic and Decor).
Gateway themed architecture: Using a special tool could change the style of a building by consuming a certain amount of Astronium.
One Planet Challenge: Complete Terra-formation without ever leaving Sylva. This challenge would require a few additions, but would have decent performance because the save would only contain one planet. This challenge mode would require a way of obtaining planet specific resources. Adding meteor showers that have concentrated mineral nodes is one delivery method for the solid resources. Gases could be obtained by scavenging crashed satellites or by making all gases present, but at extremely low quantities.
Solar system and planet customization: This would enable players to create their own solar system in steps but would disable achievements and unlock-able cosmetics.
Step one: Solar System, number of planets and singular or binary stars. (Steps hereafter will be repeated for each additional planet)
Step two: Planet Customization, number of moons, size, rotational speed/wind and sun availability and the option to tidally lock the planet.
Step three: Available resources per planet and rarity of each(this would require moderation, making it so only one resource could be primary, only one could be secondary going up in scarcity until the final resource selected for that planet).
Step four: Selection of flora and maximum soil density.
Step five: Research sample quality.
Game Flow Improvements
Console UI: The console UI could use some TLC, as controllers are a bit sluggish when compared to a mouse. Selecting a planet to travel to while they are constantly moving can prove quite difficult with a joystick, even more so if the planet is at it's maximum distance from your current location. Selecting and transferring items to your backpack is easy enough, but removing them can be slow at times. Removing placement preferences would be a convenient option also. When trying to place an item on a silo while that silo is on an extra large placement node, it will often try to snap to the node itself, rather than the silo the cursor is directly over.
I think I'll quit there, I hope that some of my suggestions help and apologize for any topics already posted elsewhere.