First off, I just want to say that I have been really enjoying the new scenario The Lost Colony. It has been really fun exploring the valley and finding the hidden locations you have scattered across the map, as well as using the various vehicle designs that you filled the valley with. I also really enjoy the economy, contract system, and shops and think stuff like that is excellent. I think these changes are exactly the sort of things Space Engineers needs more of. You could release pretty much any type of scenario for use in either single-player or multi-player, and multi-player scenarios could be either PVP or PVE. I have a list of several scenario ideas that you could possibly add into the game.
Single-player/Multi-player PVE scenarios:
1.) Similar missions to The Lost Colony but on other planets or moons, or possibly even on asteroid stations. I would especially like to see one on the alien planet or moon where you have a safe zone to keep you safe from the spiders, but when you go out to explore abandoned bunkers or mines you get attacked by them.
2.) Some type of mission where you have to travel from one planet or moon to another at some point, perhaps as part of the story. It would be nice to get to use some of the cool ships that you guys have built in the loading screens, even if I do love using my own designs as well.
3.) An actual mission with a story like The Lost Colony where you are stuck on a crashed space ship or damaged one in space and have to figure out how to get it back online or how to escape. It would be extra cool if you were something like spiders onboard or small drone attacks to keep you on your toes. I know we have the crashed red ship scenario but there isn't really anything exciting happening other than just being on a crippled ship.
4.) A scenario with a pre-built base with defenses that gets attacked by swarms of drones. I believe there is one like this already except that the area you have to defend starts off completely empty and you have to build everything yourself.
Multi-player PVP Scenarios:
1.) PVP Space Battles would be a nice set up scenario to have. There could be two bases on two asteroids a reasonable distance away from each other with a prebuilt squadron or squadrons of small ships as well as one or two armed large ships for fighting each other. The objectives could be varied like CTF or TDM or some type of objective grabbing where you have to grind down and then rebuild certain blocks to take control of them. There could potentially be projectors and welders in a part of the station along with a stockpile of supplies to allow players to build their own ships, but I think it would be nice to have some of yours ready for the start of the game. You could also do this scenario with some type of mothership/carrier instead of a base for an alternative scenario so that both sides are mobile in the PVP match.
2.) PVP planet/moon battles would be cool as well. On planets, you could have battles that were entirely ground-based (like The Lost Colony) where no one has jetpacks and there are no aircraft built unless players personally build one. You could also have other ones where there are both ground vehicles and atmospheric aircraft (or spacecraft for lunar maps). As I played The Lost Colony I couldn't help but think how cool it would be if there were an opposing safe zone or similar unsafe base on the other side of the valley or mountain that we had to battle with in our vehicles.
3.)PVP Planet vs. Moon would be a long-range, but possibly very cool scenario where one team has a base on a planet and the other team has a base on the moon directly above from the other team's base, and the two start with ships capable of traveling to the enemy's territory. It would definitely be a long term game due to the distances involved, and perhaps it would be too much for the games engine to handle (I don't know), but it would be cool having to fight over such a distance and figure out how to invade or destroy the opponents base on another planet or moon. Players could potentially rush the enemy with bombers or dropships, land ground vehicles and try to storm the base, or secretly land and build their own base closer to the enemy's main one.
4.) PVP race tracks on various planets or moons could also be cool. You guys could build some really cool tracks with tunnels and things like that on different worlds with different gravity settings that could be fun for racing around with friends. You could also do space race tracks in asteroid fields that could also potentially be fun with players.
5.) Having a PVP battle arena where you can projector build various ground vehicles or possibly even aircraft and fight other players could be fun as well.