I generally dislike mechanics that penalize you for being ahead. In a competitive game, it just feels lame (not to mention problems with realism) to punish you for doing well at what you're supposed to be doing. I do agree what others have said, however, that some sort of rubber-band mechanism might work, and specifically the AI's complete ignorance at how to use the WC is something that could be adressed - for instance, if I'm game leader and playing cultural game, all the AI's will happily applaud and vote for my suggestion for world games, even though I'm leader in production and hence will be likely to win first prize, thus putting me even further ahead in culture. Obviously, in that case AI should have a sanity check to make it realize that even if it is itself aiming for cultural game, because of circumstances it will not benefit from this suggestion.
When that's said, I do feel that the victory conditions of Civ is one of the core issues with regard to this problem. Most of the victory conditions - particularly science victory - are very cummulative over the entire game, and this supports the runaway tendency that once one player gets ahead, he'll be likely to win. I do fell the BnW culture victory shows a path in the right direction (even if there's still room for improvement) in the sense that BnW in many ways resets the game around renaissance era, which is the point were cultural victory starts for real. Certainly, you can accumulate some tourism in early game which will work as a buffer, but even if you neglect tourism early on, you can still win cultural victory if you perform well in the other areas and starts focusing on this around midgame (at least on lower difficulty levels). Early game obviously still is important, because this is where you'll lay the ground and position yourself for second stage of game, but second stage is where actual victory is decided.
I do think the other victory conditions could take some sort of lead by this. In scenarios, we have victory points that you can accumulate by various specific means, and maybe this can be transfered to the regular game - literally or indirectly, a bit like tourism works like a sort of victory points for cultural victory. I know the science-victory-through-spaceship is a stable through the entire civ series, but perhaps we have reached the point where this has to be reconsidered. One could envision an actual space race, where science victory is only reached once you succesfully engage in space race with another or several other civs, rather than just having the civ leader winning a de-facto time victory, as it often plays off. In that way, I don't see a lot of difference between science victory (reach certain tech point = accumulate X amount of science over game) and vanilla cultural victory (finish 5 policy trees = accumulate X amounts of culture over game). I don't have the exact vision on how that would play off, but I do think there's room for rethinking game here.