So broadly the difficulty of Winter Covens is so many of the Winter Rituals are really situational, Winter has some of the most difficult rituals to cast effectively. Whether that is because you need dominion on an enemy territory (which rarely happens) or a rare battle opportunity against does that are impacted by the warding rituals (Which do come up but again situational). or just being the most high mag rituals in Imperial Lore.
Winter does have a really good selection of solo rituals, but that’s less useful as a group concept, because there isn’t much to pull you together. It also has the majority of curses, so is great for a high PvP coven.
So I’ve tried to come up with a few distinct coven concepts (although generally, have someone able to solo Words of Ending, Balanced Blade and Touch of Frost, try to be able to cast Whispers as a coven.)
Cackling Witches
Take some curses, search your national brief for the reason you are actively seeking out people to curse (There are a bunch of meddling archetypes around which are probably a good basis for this group concept.) Ignore anyone who tells you not to curse. This thread talks about this in more depth
All the Herbs
Winter has one of the better economic rituals, Ruthless Vigilance, because it’s possible to cast it using what the ritual produces and still make a profit. If you are willing to do a fair bit of legwork, and arranging bulk castings of this could be guaranteed rituals each event. (It won’t make your coven a great profit, but it’s consistent game.) You probably want at least one apothecary with the mana boosting and winter ritual potion sets.
Tradition: I think magical tradition wise, I would actually try making a Totemic symbol set around plants (there is a lot of OC lore to choose from and it could be interesting.)
Winter is a Battlefield
Between most of Winters Combat Buffs being for battlemages, and a number of battlefield rituals (Wardings, blow up fort etc.) Winter Combat Coven feels like an interesting niche, many of the tricks are situational, but if you built the characters around being able to have fun as combatants as well it probably works out. (And buff each other with coven rituals.)
It’s worth bearing in mind you can switch two rituals each event, and so building this around a core of combat buffs, and then having some battlefield rituals you switch for the ones you need is probably optimal.
Items It’s alot cheaper to get combat skills than ritual Lore in items, ignore the staffs and go for the cheap combat buff items Greensteel Bracers, the cheap mage armours. The apprentice +1 spell items.
The Necrohistorians
I’ve had a lot of fun out of being in a split Day/Winter Coven (it’s probably more Day focused but Curse analysis and Whispers are really useful tools in the history game - although having Day helps with finding historical threads to pull.)
You probably want at least one character with voice for the dead, to secure your necromancy niche.