I will be anticipating the release of Elder Scrolls VI, which will be coming out shortly after finishing Skyrim. Because it took me years to get bored with Oblivion.
There will be plenty of great games to come. I am excited at the prospect of brand new IPs as well as what the future brings for some old ones. That said, I will probably be replaying Skyrim for a long time. After all, I've played Morrowind for 7 years.
After Skyrim I will be able to finally get around to playing COD, FIFA and I'll probably also have a huge craving for Minecraft again (as sad as that sounds). That could last me to the next Fallout, I guess. Hell, that doesn't sound so bad after all!
When i finish the game i will go back to normal stuff and play some other games, i can never start a 2nd playthrough right after i finish it, i will let it pass few months than ill start another playtrhough .
Play it again and again and again and again x1000, make some mods, play some mods, play it vanilla, talk on the forum, wait for some dlc's and possibly expansions, etc
It'll take a month to get bored with Skyrim. By then there'll be plenty of mods out. And in two-three months time there will be huge city and province mods. Maybe someone will make a Real-Time Settlers New Wave mod for Skyrim or a huge Telvanni continent mod like in Morrowind.