OK some achievments give you a new dye or unlock a pet, but in general they don't mean anything. What about getting an awesome ability when you max a category, the ability gets stronger as you max more categories of achievment points.

I don't mean altering the players 'powers' in anyway, just something cosmetic that will be noticed and cause you to be revered and worshiped throughout Nirn.

So what about levitation? When you max a category, perhaps exploration or crafting or even a DLC category, then you get an extra ability to levitate, to start with perhaps just two foot off the ground. As you max out more categories you can levitate higher until you have every achievment there is and can levitate a couple of foot above another players head.

I don't see it giving the player unfair advantage in PvP and it should be up to the player if they choose to levitate or not. Levitate should make any stealth ability void whilst levitating. The mechanic exists, some world bosses have it. The only hassle might be stopping the player levitating over barriers like rocks and walls, but that can't be unsolvable.

Really just a cosmetic reward but awesome cosmetic and very hard to get. Easy way - housing (pay to win?) but since that give you only inches of levitation on its own, it will become instantly obvious who is good at it and who is not so good.