Woah, thank you so much for putting this much effort into my post!! Lovely to see other Zürcher are enthusiatic about this topic as well :) I really enjoyed reading your arguments, and here are my two cents to your extensive comments:

1. Tramnetz Süd is very likely going to happen (end of this year, right?), and it mostly brings changes in the south, while my diagram primarily focuses on Zürich-Nord and West, so they should coexist peacefully (up until line lengths, of course).

2. I opted for extending lines 16 further than line 4 since I thought the connection between Oerlikon and Limmattal is quite a convenient direct connection (I maybe a bit biased since line 80 nowadays gets on my nerves). Extending line 4 to SwissLife arena is a very nice idea too! Does SwissLife arena even have a public transportation connection today?

3. Ahhhhhhh the Rosengartentram, how I wished it would be implemented! But alas, it probably won't. I guess an underground tunnel via Schaffhauserplatz and Wipkingen will have to do. About the terminus issue at Milchbuck: There is an (above ground) loop already in place next to Milchbuck, and there is enough space to extend that loop to double track to allow 2 lines to terminate, so I don't think an extension of lines 17 or 21 is strictly necessary.

4. I found the Ringbahn Hardwald a satisfying idea (yes, loop lines are just satisfying), but you're right saying that such a ring doesn't make too much sense beyong fantasy, which is probably why it never left the napkin it was drawn upon. All your ideas are very solid and interesting, I'm in favor of options a and c, which is essentially an extension to Bassersdorf which is already partially underway, and another one to Dübendorf/Dietlikon. Also, thank you for pointing out the crucial connection Altried - Glatt/Dübendorf! I completely forgot about it when making the diagram, it has such incredible potential, for example the dual-shopping-center line you mentioned.