I came up with the same problem when I wanted to rank players in each year for my book "The concise history of tennis".
Up to around 1996 we fortunately have several published Top10 lists from various experts, so I used an arithmetic average of them.
But between 1997 and 1999 only John Barrett published rankings, and even he has stopped in 1999. Fortunately, the French Tennis Magazine took over between 2000 and 2007, but even they have stopped after that (although in 2010 they once again published a ranking).
So I myself came up with a point system, where I decided to take only the best 5 results of players. I also took into account the ITF and WTA awards. My rankings for 2004 (taking into account the French rankings and the ITF and WTA awards):
1 Sharapova, Maria
1 Myskina, Anastasia
1 Henin Hardenne, Justine
4 Davenport, Lindsay
5 Mauresmo, Amelie
6 Williams, Serena
7 Kuznetsova, Svetlana
8 Dementieva, Elena
The French Tennis Magazine co-ranked the first 3, Sharapova and Myskina got the WTA and ITF awards, respectively, and Henin and Sharapova finished tied in my point system, just ahead of Myskina.
So I co-ranked the first 3 in my book, but I put Henin below the 2 Russians as she won no award that year.
I think we should extend the Blaster's Panel rankings (compiled here in this forum for years 1958 to 1975), and do rankings after 1996 (or even after 1990).
Otherwise further generations will be left with the WTA computer rankings, which are certainly unfair for a great number of years.
I suspect that if there were TENNIS and World Tennis magazine rankings for 2004, they would have probably been something like this:
1 Maria Sharapova (magazine rankings tended to favor splashy, and popular, winners like Sharapova. Especially when Wimbledon is among major wins. The YEC didn't hurt her either. And would be enough to offset the rest of her record for the year.)
2 Justine Henin. The player who I think deserved to be #1 for the year. The Australian Open and Olympics win were huge. But her 2R and 4R losses at French and US, and w/d from Wimbledon would have hurt her overall chance.
3 Lindsay Davenport. Consistency would have been rewarded with a #3 ranking, despite no major win.
4 Anastasia Myskina. The French win was glorious, and leading her country to Fed Cup helped too. Then there was her 2R loss in the US Open, among other bad results.
5 Amelie Mauresmo. Pretty solid year. But just not enough to push her any higher.
6 Svetlana Kuznetsova. US Open win was stunning. French Open collapse vs Myskina proved even costlier than though of at the time.
7 Serena Williams. A dominant player only 18 months prior to end of '04 season, she did reach Wimbledon final. But indifference and injuries kept her away from the pinnacle of the sport she had dominated for a short time.
8 Elena Dementieva. Surprised everyone by reaching 2 major finals, but folded in both.
9 Jennifer Capriati. Part of the year's biggest rivalry (w/Serena), winning twice. But not much else from former #1 to warrant a higher ranking.
10 Venus Williams. Like her sister, injury and indifference caused a steep drop in her performance.
Although Davenport also ranked as year end #1 in 2005 (
) I don't think a real #1 player emerged until Justine Henin in 2006. We think we're going through a soft patch now at #1. But from July 2004 through most of 2006 the WTA did not have an active, truly ensconced #1 player.