Yeah, there is no shortage of people doing this in C# and XNA. My original concept was to do this, actually, for the sole reason that it wouldn't be too difficult. If you think about, using XNA as a base, you are already about 90% done with the truly difficult stuff. All you really have to do is create an editor with C# (with the exception of map editing, easy), and embed a scripting language (also easy once you do some research).
I am really glad now that ARC is being built from the ground-up. All these other editors appear to be a dime-a-dozen, and are platform dependent on Microsoft systems.