This is just a thought experiment more or less. Personally I don't like AP, and without artifact weapons it seems like a good chopping block moving forward to the next expansion. The other side of this is I don't feel like the high end reward model in raiding or in M+ is appropriately rewarding for the time investment.
Loot item level, Titanforging, and Ilvl Upgrades
While this has been a controversial subject in Legion, I'm mostly ok with it. In regards to titanforging I think there is an extremely simple fix for most of my concerns with the system.
-1. Increase the item level gap between heroic and mythic level raiding
-2. Make mythic raid loot closer to the TF cap.
-3. I'd like to see the base item level of M+ loot scale at higher levels. For example right now I think a +15 should jump to 920 gear, and a +20 should jump again to 930 gear. The point being if you are able to clear the extremely high levels, it should be worth your time.
So that brings us to the last part of this. Upgrades and how do you keep the content rewarding in the longer term. I'd like to see the old currency returned that we've seen in previous expansion that would allow you to upgrade items to the current Titanforge cap.
-1. It would be awarded based on your highest M+ completed for the week, very similar to how AP is awarded now.
-2. Allow the final boss of the current raid tier to drop, scaled based on the difficultly you defeat them in.
-3. No weekly cap, simply limit what awards it as seen above
That would keep mythic dungeons rewarding in the long term, you'd always have a reason to do it, and it would combat loot RNG. While you'd still have your weekly item (I believe this system can be unchanged), if it was a bad item (usually) at least you'd get the currency to upgrade something that you are wearing. Also as a direct replacement for AP it provides a measurable constant increase in power over the long term, while allowing players to combat Titanfroge rng in those unlucky slots. I've loved the M+ system, I just don't feel like the reward structure has been very good for it on the higher end. All of these would reward you for completing higher keys.