Let me first try to do like I wrote, if after testing you find it good/useful, we can talk about more features.
One thing I can predict: Zoom in/out = ATM I have no skills for that matter and I really dont know if I am willing to learn such stuff :-)
Why I want to first get it tested by you (or others) before talking about additional features:
I have a bad memory in my mind where a User needed an application,
I was already 80-90% done with it, without sharing my application, just showing screenshots,
more and more wishes came in, more and more people did their positive and negative feedback,
up to a point where I lost my fun in producing it = deleted project.
To not let this happen ever again is the reason why.
opacity = no problem
temporarily hide = here I need to re-ask at given time how you mean it.
zoom in/out = problem
annotation = here I need to re-ask at given time how you mean it. (I guess a picture say more than a thousand words....)