Since July 2011, I have been working on a new engine in C# called Mirage XNA using XNA 4.0. The point of the project was to rebuild the idea of Mirage Source (a online 2d Open Source RPG engine) but providing a much more powerful and easier to use base engine. In reality, Mirage Source 4 wasn't much more than an open source game - there was hardly any abstraction from the engine and actual game content. Working with the engine, too, could be quite troublesome due to the extreme measures Mirage Source 4 took to squeeze out every little bit of performance.
Mirage XNA aims to change all that. Like Mirage Source 4, it isn't a "game maker" - programming is still required and you can't rely on others making all the contents and features for you. But it is far more flexible and easier to use than Mirage Source 4 ever could be.
Mirage XNA is ready to be used, but it is strongly suggested that you don't actually use it for your own game. There is still quite a lot of work to be done on the engine with bug fixes, improvements, code refactoring, and a few features. But you can help speed up the development of Mirage XNA to bring it to a more complete state by joining the development team.