Just started playing D3 again, beat Inferno and am Paragon level 3. Is there a more challenging mode now. Can I make it harder?
Options -> Gameplay Options -> Monster Power
It lets you set the "power" of the monsters, very much like typing "Players 8" in Diablo 2. It gives monsters increased HP, damage, while giving you increased magic/gold find, and increased chance for monsters to drop items:
You'll want to gear up a bit before taking on MP level 1, and if you find it easy, move up a level, etc. You should farm MP1 though, because it makes all monsters level 63, giving them higher chances of dropping ilvl 62/63 items.
One reason for WoW decline I often hear is that its an old game and naturally people get bored with it. But TV is much older and people still watch that. Surely if a thing you once enjoyed keept staying fun, you would still use it?