For new types of crops, my thoughts are that maybe they should be a bit more than just a crop type that just gives more food, or something simple like that. They should also have an interesting way of being purchased, like something for a higher amount of gold at the Trader (like Relics).
There should be prerequisites for their availability, like a sustained amount of gold stored in your Trader building indicating you have reached the point of affording them. This would be especially useful if there was an opportunity to get one or two types earlier in the game like in Tier 3 (the types that can be planted in Fields only, at this point). They should then come every once in a blue moon once reaching Tier 4.
If new production chains are added later, this mechanic could be a part of how you get the crops for them (like Grapes, if Wine is added to the game).
Some ideas for new crops:
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Barley: Has all the same growing stats as Rye. It gives a percentage of bonus Barley on top of your crop harvest, which can be used directly by Breweries to make Beer in greater amounts per Barley than regular Wheat. This effectively gives a boost to your Wheat harvest, but only if Beer is being produced.
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Lentils: A Bean producing crop (unless family is renamed to Legumes) that gets a bonus percentage of harvest from being grown in areas of lower Environment Fertility Factor (though will still be just as reliant on Fertility). This is designed to be a big boon in maps like Arid Highlands.
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Beetroot: For a slightly higher impact on Fertility, Beetroot is a Root Vegetable that come with a percentage bonus harvest of a Beetroot crop, which can be used directly by Apothecary Shops to create Medicine at a higher rate.
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Lavender: Produce Lavender that can be used instead of Herbs to produce Soap. Fields growing Lavender give a large bonus of Honey production to Apiaries covering them. Chance while being grown of spreading foragable Lavender plants in range, which also can spread new Lavender plants.
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Strawberries: For the Arborist, these are single-tile plants that produce Berries. They must be replanted each year (more labour intensive), but produce high amounts.