Hopefully you don't mind if any of us join in.
I'd like to see...
A tilemap system with an api allowing very straightforward (the same as the rest of the api) loading of Tiled maps.
The system should handle loading an atlas image containing the tile images. Perhaps one atlas image per display layer in the tiled file.
An api call to make a given layer visible or not. Active or not. The data for the layer should always be available for us to read or change regardless of its visibility and active status.
The system should handle multiple layers which are depth sorted and an api call can set the depth of each layer. These layer depths work the same as the sprite depths making it easy to have enemies and objects appear in front of one map layer and behind another layer.
Any cell in any layer can be changed with a simple api call to specifying the layer, x,y and new TileID.
Collision checking should be available by simple api calls that check a sprite's collider against a specified layer. In this way collision layers can be easily defined in Tiled. In fact multiple layers can be defined such as platforms, walls and ceilings in one layer and doors / warp points in another layer, etc.
Automatic optimized rendering of each layer should be handled by the system. Again api methods should be available to set the tilemap rendering frame size. And a method to set the position of a given layer.
This one thing would be a huge HUGE boost for making 2D games whether small single screen or flip-screen platformers or large scrolling metroidvanias or top down twin stick shooters or top down rpgs with multiple areas in a huge world.
Basically the easier this kind of core stuff is the more ambitious projects people will make in AGK. And those games end up serving as the best advertising for the product too.