I am currently working on a converter to create stages for our Nintendo Virtual Boy game engine (VBJaEngine) using Tiled. Thank you Thorbjørn for creating such a wonderful tool, it makes life so much easier! 
In my maps, I make heavy use of the custom properties and I love the object type feature and its default properties. Would it be possible to extend the object type feature to also support layers? Maybe even tilesets or terrains?
It’s great to hear Tiled helps you out creating content for Virtual Boy games! I had a chance to try a Virtual Boy once when I was a kid. Maybe with the coming of VR it will be popular to emulate? 
This is definitely something I want to add. Indeed only objects have this “type” field on which the predefined custom properties are currently based, but it should really be possible to pre-define properties also for all other data types. I had already mentioned this at the bottom of the custom properties documentation (which I see also mentions some things that have been added in the meantime…).
I have opened an issue about it to make sure it isn’t forgotten: