I used the tilemap object. I've never needed any particularly sophisticated tools, so maybe my case isn't a good example. In the game I'm thinking of, IIRC, I had auto-tiling for a couple tiles types (not gonna lie, that was difficult since I hadn't dealt with bitwise logic previously), plus copy/cut/paste function, and horz/vert flip. That's about it.
Unfortunately, that code is not very modular (early project), so I can't exactly strip it out easily for a demo capx. Also, I never worked on a way to migrate levels back into the editor, which, I can imagine, would take some doing.
EDIT: Okay, so here's an idea: A "Load Tilemap" option in the Tilemap object Properties. It let's us specify a project file that contains JSON representing a C2 tilemap. Then we can create tilemaps with an in-game editor, export as JSON, and bring the new layout back into C2 with minimal fuss. Couldn't this be done within the scope of C2?