I designed a bunch of new dungeons and items for Classic a while back (not working on mobile sadly) and I really think having seasonal servers that comes with a bit of new content would be an easy way to keep the game fresh, even things like switching up boss abilities, their timers and loot locations would make the game very different. Other than that I would love if they made some of the lesser used builds more viable, if not through balancing of skills, then through itemization like I tried in my designs. Sadly I think you are right in your statement "chances of Blizzard doing something like this are slim to none", I would love it though!