Guess that many with a lot of CP are doing this already in PvE quest lines, but mention it anyway:
After a year pause from ESO I returned only to discover that normal NPCs are insta-killed by my 420 cp Nightblade in the Thief and Brotherhood DLCs, so decided to create my own scaled-down armor and weapons, and not use food buffs. A bit interesting to experiment with bought or rewarded weapons too, stuff I formerly discarded because I always wanted the best gear.
Also try out weak spells and abilities, not your most powerful ones.
Guild Wars automatically scale down characters when they enter lower zones, so why not do it yourself in ESO, to make it more challenging.
But be prepared to get a bad reputation for being a horrible player when your 500 cp toon has problems killing targets on a much lower level