Harem pants are pretty amazing. Batik lounger pants are as well.
Hands down, wrap pants and sarongs are possibly the best summer wear. At least, in Florida they are. They are really versatile, generally lightweight, and cheap. That, and they are really easy to make yourself. I swear that every person who lives somewhere hot needs at least one pair.
Back on topic - well, how about lighter fabrics, like thin track suit bottoms? They're usually more baggy too so might keep you cooler. Cotton is meant to be cool or so I recall.