If you want Ice Cream Sandwich/Jelly Bean support on your HTC Desire Z device, you must email HTC and Qualcomm:
http://www.qualcomm.com/connect/contact (more info in the 16-1-2012 Update)
With this being talked about now, what will your realistic Project Ara phone have?
Mine is pretty simple:
Qualcomm Snapdragon 800 2.3GHz
Adreno 330 450MHz
2 GB RAM
3.8 V 2300 mAh, 8 Wh, Qi wireless charging
Slide out QWERTY phy...
tl;dr everyone has a different answer
One of the main reasons I have not upgraded to Nougat, is the lack of Xposed support.
And the wait is over: