I admit, the transition will not be a small process. Like you pointed out, the discussion section and, even worse, the reviews must be merged. The current ratings would have to be converted or reset. Overall the change would have a big impact on the site in general. Among others the watch list features would need to be adapted ( eg, how to deal with new seasons in the plan-to-watch list etc).
However, the number of dramas with their stories spread over multiple seasons seems to be continuously increasing (at least that's how it appears to me). If this is true though. it would be better to start planning for this now, when there is still a relatively small amount of dramas affected than waiting a few years when maybe every other new show has two or more seasons.
Of course the work load for this would be huge. Maybe it would be easier to come up with a temporary solution first that isn't as complex and doesn't have too large of an impact on general site operations.
One idea would be to link ratings of different seasons (again only for those that don't have independent story lines). So when you add the second season to your watch list your rating for both seasons is identical. When you change the rating for one season your rating for the other season is automatically set accordingly. Of course this only works as a temporary quick fix and is far from ideal, but it would be a start. Just an idea though, there are probably other solutions.
