Winter’s ability to turn you from an everyday ritual mage into a pretty good battlemage (once you’ve got mage armour and a staff) I think is a fun thing to run with.
Battle Mage Rituals
Ravenous Tongue of Entropy is a cool upgrade (cast the spells shatter, weakness, and paralysis as if you knew them) for only 5 mana mastered and has the hilarious roleplaying effect “You feel an urge to lash out with your magic at anyone who crosses you, questions you, or obstructs you.” Lean into that for maximum fun times 
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I had some fun with Crumbling Flesh and Withering Limbs, it’s a cheap way to add some combat ability to a mage alongside carrying a buckler which anyone can do. It can be a way to power up before a skirmish so you’re not just a squishy mage. Also no one expects a mage to be able to cleave with their rod, so it’s excellent for disarming or dropping people before you drag them out of your tent for rudeness.
Hungry Grasp of Despair letting you cast weakness 4 times a battle/skirmish (if you go and recharge in a magical aura after) for 7 mana or 14 measures of Beggars Lye is also pretty boss. Also has the brilliant roleplaying effect “You feel powerful and deserving of respect. You experience an urge to punish people who disrespect you or your authority.”
Then there’s pretty much the best Winter Ritual for battle mages, Coil of the Black Leech with the roleplaying effect “Whenever they gain hits due to this enchantment, they feel a rush of pleasure. Once they have used this ability once, they feel a constant urge to use it again even when they are not injured.” 10 Mana when mastered, but only an extra 7 mana for an extra coven member if you’ve got the power level. Then simply add a Splintering Gorget to taste. You too can live your best vampiric wizard life with this ritual and perhaps your whole coven can too 
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Oh yes and there’s Devastating Scythe of Anguish and Loss, not just a super cool name for a ritual but it will let you call IMPALE with a staff at will as if it were a 2pt spell. With this spell you can ruin the day of a lot of Barbarians and it’s almost just worth it for the look on people’s face when you, a battle mage, busts out Impale after Impale in a swirling orgy of destruction. Yes you will need the Battle Mage skill, some extra mana and almost certainly a lot of Mage Blood potions but it’s worth it 
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Also has the roleplaying effect “You have a sense that everything around you is dying, crumbling, or rotting. Living beings are going to die, and everything will eventually fall apart and be lost thanks to the relentless march of time. You find it extremely difficult to appreciate beauty, seeing the eventual decay into which all things will eventually fall” for extra goth points.
Personal Enchanments
There’s also some pretty cool enchantments which you can hand out to warriors around you.
Fight Tooth and Nail gives people two uses of Unstoppable a day without spending hero points at the cost of 6 mana mastered. And y’know the roleplaying effect of “You find it difficult to show mercy to your opponents; you feel an urge to execute fallen enemies on the battlefield, and to pursue fleeing enemies until you can beat them down and execute them. In social situations you are likely to lash out at anyone who you think is threatening you or your friends, and will feel the urge to beat such people down and then execute them” is really just the cherry on the top 
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And there’s Pallid Flesh of the Dead King which gives three ranks of endurance at the cost of about 7/8 mana mastered, oh yes and the ritual target getting a venom which can’t just be removed with purify. But cool people don’t worry about that sort of thing and you’re cool right?
The other fun bit of this ritual is you can substitute ingots of Orichalcum, Weltsilver, and Tempest Jade for mana at a 2 for 1 ratio. You just need to make it really obvious that the ingots are a tribute to the Thrice-cursed Court. But I’m sure that wouldn’t cause any issues.
Cool Battlefield and Skirmish stuff
And finally as @McGonigle mentioned there’s some battlefield stuff that’s situational, but very useful when they come up.
Pakaanan’s Iron Shutters which seals a portal for ten minutes can be just what a battle, skirimish or quest needs to achieve an objective.
Likewise Ward of the Black Waste, which wards an area against Vallornspawn can be intensely useful if you’re going up against the Vallorn.
I understand that casting Inevitable Collapse into Ruin on a battlefield fortification makes you feel like a superhero, it’s kinda like this .
And getting off a casting of The Grave’s Treacherous Edge vs Vallorn and especially Briar Husks is epic.