Although I enjoyed The Elder Scrolls Online, what I disliked most was its character classes and skill system. Having fixed character classes with class-specific skills goes against the best of what the Elder Scrolls series is all about: be what you want to be. In Skyrim you learn the skills you use. If you want to be a warrior you wear armour and use weapons, if you want to be stealthy you use lighter armour and sneak a lot. If you want to be a mage you buy or find some spells and you use them. You can use any combination of skills. And the skills in Skyrim were also much better, with a level that means something and with perks that were significant and cool.
I do not want Elder Scrolls 6 to be a single player version of ESO. No way.
And yes... it should be single player, because an MMO is almost forced to follow a simplistic class and skill model because of the need to balance everything. I much prefer complex character and skill systems.