One that I've wished existed for ages would be a space-based game that offered complete customisation of your craft.
Sure, there are lots today that advertise such a thing, but what they give you is a few basic hull layouts, upon which you pick and choose your equipment to fit into a fixed number of slots.
What I'd want to see is you're given nothing but a list of basic parts in different varities (e.g. engines, guns, hull wall sections) of which you can use as many as you want, scaled up as much as you want (within resource limitations), complete with even the inner layout of the ship being at your discretion. You would design your own control panels and displays, with the links between the equipment and the controls designed by you. The basic game would be little more than a physics engine, but it would support a scripting language so designers could set up very complex behemoths. Ship design would allow you to create potentially devestating new types, which can then be uploaded for others to download and use as they want. Since the control schemes would be designed as the ship is, it would allow for ships that handle in completely different ways. Because skilled pilots might be rare, it would open up multi-person ships, where one person flies, and other man the guns.
The controls and interaction would be based off a person walking around, with few basic functions, but as soon as you use a piece of equipment, some of the buttons gain additional purpose, which are used to interact with that equipment. The player would be (mostly) walking around the inside of a spaceship, jumping from one control panel to the next to operate relevant parts of the ship.
Because the game would essentially be a big physics engine managing loads of basic parts, it means that ship damage would be interesting. If someone blows a hole in the hull of your ship, you're going to have to either seal it up fast, or find breathing equipment to use. If you blow a hole in the enemy's hull, you can then lauch a boarding party, who can storm the corridors and take them out when vulnerable. When designing a ship, you'd have to choose where you want the crew members to be - your skilled pilots and the like you'd want out of harm's way, so deep within the ship, but that would mean they're relying on sensors to give them information, and if the enemy can disrupt the sensors, they're be blind and therefore useless. Redendancies would therefore be optimal, but they'd consume extra resources so you'd only want backups on your biggest ships.
To avoid someone covering every inch of their ships with weapons, they would each require a power supply to operate. Sure, you could still cover everything with guns, but you'd be able to fire them once and it would take an age before you could fire a second time. If your power supply was limited, then you'd have to choose which systems get priority and how much.
The scripting language would be simple to use - each individual ship component would have a few basic commands, and each input would have a script that activates these commands. For example, I might have a button that turns off thrusters A and B, sets thrusters C and D to full strength, and turns on light E on the adjacent display.