Fellow sartorial blogger Will has published a useful post on essential odd trousers on his blog, A Suitable Wardrobe. This is tremendously useful advice for anyone seeking to expand his clothing repertoire, especially MTM and bespoke.
The only thing I would add is perhaps a hardwearing fabric like whipcord or venetian for frequent travellers and lightweight cotton needlecord for summer use. Cavalry twill is also a classic hardwearing fabric, which Will mentions in his post.
Incidentally, I’ve ordered recently a light grey flannel that Will decribes, as well as whipcord and venetian trousers (both John Hardy fabrics).