I am subbed exclusively for Classic, but I would download and play Retail again if these conditions are met:
1. Get rid of endless, no-lifer grinding mechanics such as AP, azerite, essences. Get rid for good.
2. Massive talent overhaul. The current talent system is awful (the only negative thing I remember about MoP honestly), it doesn't feel engaging and most of those talents are useless.
3. Retail becoming alt-friendly again. Maybe by introducing account-wide reputation and campaign progress.
4. The re-introduction of PvE tier sets and master loot.
5. The re-introduction of PvP real gear, stats and vendors.
6. Massive un-pruning. As a shaman main, I really hate to see my precious totems gone, except for 2-3 situational totems. I'd love to have 20-25 abilities you can use ocassionally, like in the good old times.
If these conditions are never met, I won't touch retail again, not even in expansion launches.