You can't have difficulty sliders in multiplayer games for overland content. You can have instanced content with different difficulty settings, and we already have that in ESO - it's called normal/veteran/hard mode dungeon/trial/arena. Overland, delve and public dungeon mobs and bosses are easy, and even dolmen bosses are pretty easy too. The only ones which are challenging are world bosses, but those aren't meant to be soloed. They have not only been buffed in terms of health and damage, but most have adds now; specifically to require more players to take down. Veteran dungeons have been buffed a lot, and normal ones have also been buffed slightly. The slider is basically the content you choose to play. One can argue the slider is not smooth enough, but not that it isn't there at all.
Of course you can have difficulty sliders - it is very simple - a factor, which is used on the end stats between for example 0.8 and 1.2 - this would alter the character between 20% lower and 20% higher stats. It is that simple.
This would lead to that an easy mode player would have 50% better stats than a hard mode player - and just 20% difference to the average one. If content is too hard, one could just move the slider towards 1.2 and if it is too easy, towards 0.8.
