Aside from adding unacceptable amounts of RNG and one-shot mechanics (or maybe absurd enrage timers), I don't think WoW can get any harder. Remember, the game isn't actually any easier or harder than it has ever been -- it's just that everyone is used to the mechanics now, and we have addons that make them even easier to deal with.
Vanilla WoW was only difficult because the game was still new and proper addons were still being dreamed up. Well, that and balance issues. The game can certainly become easier, but there is a difficulty ceiling that cannot be overcome without pissing off the majority of raiders.
TLDR: I wouldn't make it any harder. I'd try to figure out new and interesting mechanics, but even those would be assimilated within a short period of time. Just try to keep the game fun and keep the difficulty where it's at.