GDevelop now supports tilemap painting with collisions. Read the release notes here: Release 5.4.207 · 4ian/GDevelop · GitHub
I think it would be great if you could “paint” tiles. Hold the mouse button down and fill with selected tile into each empty grid square the mouse goes over. Would need at least one option set, though. To place only in empty grid, replace tiles, or place over existing tiles.
Another idea would be able to “draw” scenes. I had played a game in the past where making levels was essentially an image file for visual part and another ‘mask’ image for bounding areas. That one worked by being monochrome, with black being solid bounding areas and white being empty areas. This would be great for my racing game, as I could make nice smooth incline/decline curves. Alternatively, have it so we can ‘draw’ bounding boxes directly in the scene creator.