My girlfriend really loves Skyrim. I liked it a great deal as well, but over time I got bored with it. She, however, still plays it today. Creating new characters and role playing it in several ways and such. What she really likes about it, is that she can choose to do things in any order she wants and go to certain places whenever she wants. It allows her to create a character that she can roleplay. For example, when she creates a mage, she often goes to the college of Winterhold (I think it's called) first. When she creates a stealthy chick, she goes to the Thieves Guild first. Sometimes she switches up her playstyle midgame.
Would the Elder Scrolls Online be a good game for her? I'm thinking about buying it for her and paying for the first two months or something like that. But I'm not sure how the game is structured. If it's traditional MMO style than she may not like it as much as Skyrim I think. Just because she'll be forced to play certain parts first and is stuck to a class and/or abilities.