- I consider the last 100 games (or whatever the threshold should be) a good limit,
- with superior weight for the recent ones (say +++ in the last 2 months, ++ from 2 to 6 months, + from 6 to one year) and a full expiration after a longer period (say a year).
It's just a mix between different ideas widely brought to the forum, though this way active play is still more rewarded than past ones and expiration of active periods is not so hard to accept. Moreover, in order not to limit abuse on the top of the ladder with players wanting to stay high with no addition of games, a player would stay in the ladder only if he plays above a minimum number of games each sliding month, like 4 games started in the last period of 1 month or something like that.

And I'm a big fan of the idea of, well, "Masters series" if you may say it so : periodic tourneys of high-rated players, either RR of elimination (RR if not too large, with 10 it doesn't seem too demanding), possibly with a link to the tourney on the home page...

Afaik, last 100 games is not possible. Then a game would at a certain moment count for one of the players, but not for the other, which would bias the rankings.