There should be little for solo players to do (harvesting/crafting/housing and maybe some smaller things to do solo). As a solo only player, I moved onto other genres and found FAR better games that fit a solo only person. And if I want to play with my friend in a game, we find far more fun in games like Civilization 5, starcraft 2, Heroes of the Storm and that kinda game.
MMOs greatest AND best difference with any other genre? The massive amount of players one can interact, socialize and do things with that no other genre can compete with.
What do games like Skyrim, and the other games have? Generally a smaller, but better quality and better game suited for 1 player to maybe 6 or 8 players. A game tailored to the players experience. And skyrim has tons of mods as well that add tons of lands/quests/dungeons etc, so you can make a pretty big game to a game world all to yourself.
MMOs however have moved heavily to a solo focused experience. However that is the greatest weakness in the MMO genre. It completely erases the greatest thing MMOs have, a social and group focused genre. And this is coming from a person who doesn't like grouping at all and will never use voice chat with strangers. But, even I see that while MMOs are not for me (that is like me being a square trying to fit into a circular hole)...and find other genres far to a better advantage for a solo or (very) small group atmosphere.
There are MMOs coming out that look to do the old way, like Pantheon is a big one that goes back to the way MMOs were and should be...a group focused, social massive experience one can't find in any genre. That is what MMOs should be.
And this is coming from someone who would never play Pantheon, but Pantheon I really hope is successful because it is playing to the greatest strength of MMOs...social activities, group focused atmosphere and that is what MMOs are about
My Skyrim, Fallout 4, Starbound and WoW + other game mods at MODDB: