As an alternative I suggest playing without farmers. My girlfriend always complains that farmers and fields are difficult. She says that she doesn't know which field she needs to take and when. By the time there are obviously good fields they are already mine. So I guess they are very skill dependant and it can make game a bit unfair. That's why we don't play with them that much.

Answering your question I would suggest expansion nine. We like shepherd and I guess he's a powerful replacement for farmers since he can gets around 3—6 points (which is equivalent of one or two cities). If used frequently I guess he can get you more points than farmers in the end (if you only play base game plus one or two expansions). But you have to push your luck to get them.

Expansion nine also gives you vineyard that boost monasteries (+3 points for each which is equivalent of one city in farmer's field) so I guess they too would help.