Whiteclaws Unfortunately that's not very practical in C2. There's no file i/o (and project files are still internal), you can't retrieve object names, variable names, nor variable index, there are no listboxes and so on, and image offset was not carried over from CC so forget tilesets unless you want to use sprite objects (but then you get performance issues and zero customization). Making your own would be the way to go seeing as it would be external and open for team members or players, but C2 doesn't provide everything you need to get the job done.
Now, that's not to say you can't make a level editor for a very small, basic game, but even then you'll have a very hard time using it (and team members can't use it unless you load levels from a server, which means you can't load levels in preview.)
Also if you use tiles then you will get seams. I've tried pixel rounding, point sampling, and ^2 size textures but they still show up on some machines.