Some Questions:
1. Is it allowed to make a “leave a number/message for the next one”, or does it count as chat?
2. Is it allowed to save a certain picture on a project, then everybody can see it?
3. Is it allowed to let users change global variables as they'd like? Such as somebody changes it to 41, and the next player coming sees 41.
Please answer….

When you say “Global variables”, I assume you mean cloud variables, correct?

Well then, I can answer your last question:

3). Yes it is possible, though users can't set any cloud variable to text because cloud variables can't store text at the moment. That feature was added in to help prevent chat rooms.

With regards,

ErnieParke