I was out collecting rowan berries just the other day to make some light mead and jam, and it occured to me that rowan berries are missing from the game. Although they are quite bitter and mildly poisonous if eaten straight out of the tree as soon as they become ripe, after the first night frosts or after boiling (or throwing them into the freezer for a day in modern times) most of the parasorbic acid that makes them bitter is converted into sorbic acid, which makes them less bitter, and the mildly poisonous substances present in the seeds are also destroyed, especially during cooking.
In game terms, rowan beries could be something that's still available in late autumn/early winter when all the other berries are already out of season (somewhat similar to cranberries). The rowan tree and its berries have had a prominent position in folk magic and folk medicine, so it would somehow feel quite appropriate - would probably require quite a bit of work to add, though; coding the rowan trees so that they can have berries in the first place, plus some new graphics.