Harder mobs would make it harder, sort of... Things die insanely fast, on my Monk I'm leveling I pull 4+ mobs and don't drop under 80%, doubt it would make it harder.
More xp =/= harder. Just longer. You're confusing time consuming with difficult.
Again, no LFR/LFR makes it longer, not harder. Difficulty of the dungeons would be the same, would just be harder to find people to do them.
No crossrealm would make it easier, all crossrealm does is make farming/leveling harder because you have more people to fight for mobs.
Guards are already irrelevant, they die in like 3 hits.
How would increase cast on flying mounts make it harder...? Part of PvP is running away, believe it or not, if you know you can't win. All that's doing is making it so you die automatically with no hope of escaping at all.
There would only be 1 of these servers, transfers would be unnecessary.
All gear would drop? Again, that doesn't make it hard, just makes it time consuming to get the items you need. You're again confusing difficulty with time sink.
25man raiding, eh, doesn't matter. 25 man isn't harder, just more of a hassle to find people since you need more than twice as many. I'll let you have this though, since your other ideas are kind of pointless.
Anyway, there's no real need to make it harder. People need to realize, WoW was never hard to begin with. People were just bad at it, they've improved, it got easier. But WoW was never hard, vanilla was a massive grind fest, TBC was fun but not really difficult, and people have been saying it's been getting easier since TBC. Not too sure why people think that, it's the same as always. Moving out of fire was never hard, it's just now people are completely used to it so the mechanic is no longer interesting.