The reason ATP only count 5 tournaments is to protect and prevent people from over-playing too many 3rd or small tier tours, which is unfair to others. Hi Stuey, Blacky, and everyone,
Actually, based on what I read, I still think that following the real ATP tour calendar is the best solution.
But, let me say that your idea of 2 seasons is not bad. However, I think it still needs to be worked on. For example:
1) Since we only have half a year to play the season, we can play many small tours (besides the 4 Grand slams and 9 Master Series). But, we need to only count the best 3
2) If we choose to keep the 08 rankings and rollover to 09, how can we implement the "defending points" systems in 09?