Awesome Tutorial! Thank you for making this. It's crazy how complex this gets.
Here are a few more ideas to add to this tutorial series:
1. Complete Depth System (To fix some current issues)
2. Interactable objects that can go on block objects (For example, grass, tree's, pots, etc. Maybe Buildings too. Just to show how to handle placement of these types of objects in the room editor.)
3. Enemy Pathfinding (Enemies that can jump over a block if its short enough and even navigate up a ramp if it exists)
4. Tileset Collisions (This way, instead of having block objects, you can just place cliff tiles. This video tutorial has something similar:
My GameMaker Template for 2.5D Platformers Explained (Thoroughly)
)