CEO of Keen Software House has made a blog about the roadmap for Space Engineers and Medieval Engineers.
You can read the whole blog here. This is some kind of short brief:
Space Engineers is entering the third year in Early Access, the phase where (nearly) all necessary parts have been assembled and we are ready to move into a major polishing and bug-fixing period. This will probably be the longest and final bug-fixing period before the game is released in its final state. Our top-most priorities are to fix all annoying issues that make the game hard to play for some of you, improve the multiplayer, and polish the game.
Medieval Engineers will complete one year in Early Access very soon. Many of the features that we planned to implement are already in the game – e.g. survival mode, castle siege mode, multiplayer, simple AI characters, and crafting. However, the overall gameplay lacks a polished feeling, and control and gameplay aren’t intuitive. These are the things we will improve first so we can continue adding new elements, like planets for example, to unify the technology we use for both of our games.