How about extension that could load and display maps made using Tiled editor? Tile-based editors are good for things like platformers and to my reasoning making extension that can load Tiled maps instead of implementing tile mode would be easier to do.
Now I do realize that Tiled maps are in essence XML files and theoretically could be implemented using XML extension, but I’m sure such implementation would be clumsy.
//edit: Since Game Editor is written in C++, perhaps this can be of any use.
You could use the already written save/load functions to achieve this.
You can already do that, but using a lot of sprite object with texture for each cells (for example 10 000 for a 100*100 map) will reduce the performances a lot.
So, this feature is a good idea (using one texture in an object that display a tile map).
Yup, though you’d need also some function to check with what tile specific point on map collides with, so you can do things like playing different walking sounds when on dirt and different when on rock.
Also it would be good if you can split layers into separate sprites, so you won’t walk over trees or buildings 
.