This made me think of various computer rpgs which make you pick an ambition (or synonym) for your character: an over-arching goal of something you want to achieve in life - and then what would be good legacies for magi. From the thread above, we already have two: books and apprentices. What are others?
I can think of a few. A Covenant (newly founded, rescued, or well-stewarded) is the obvious example. A mundane family is another. Then there's things like a particularly famous item (maybe even a talisman), founding a mystery cult, cabal, or lineage, or (for the really ambitious) a new House or Tribunal. Any others?
A breakthrough -- like being able to do Certamen with three or more participants; or a season-long 'Wizards Vigil' that lets mages cooperate on non-spell (think book-writing or 'craft magic') activities; or fully integrating known magiks into Hermetic Theory....
For Verditius there is obviously a powerful enchantment.
Founding a mystery cult
prevailing in some legendary feat (especially for Flambeau)
becoming immortal
I'll break them up in to different categories. Many of these will completely mess with game balance and the world setting, thus they really should be end goals.
Societal Legacies
Have magi accepted by general society.
Have the church officially approve the order of Hermes and magic in general.
Improve relations amongst the major religions, reducing the risk of religious wars and conflict.
Assist in Re-establishing a grand empire that provides the degree of order the Roman Empire provided.
I'd argue societal legacies are the hardest, and some may say my suggestions ridiculous. Magi have long life times, and we are talking about legacies.
Order Legacies
Forming a well attended apprentice school, which would greatly improve the orders power (200 XP in 2 years is not unreasonable).
Integrating mysteries to the order. Controlling twilight, crafting items well, fairy aura being good, mystery cults, awesome. Bonisagus shared his brilliance. Why is the order so secretive now?
Improve relations amongst the houses. Tytalus and Tremere are ready to throw down at a serious enough provocation. Any big Fae conflict, Merinita get the Diedne treatment.
Great magical Achievements
Improved Mercere Portals. Modifying Mercere Portals, so that the receiving end knows who is trying to travel through the portal, and has to accept the portal being activated before someone can travel through it. (this could be considering an order achievement)
improved Longevity potions - No sterility. Improved potency (1 pawn for every 6 year, +1 for every 4 lab total, etc)
Removing the gift penalty, without removing the gift.
Improving parma and/or Aegis. Parma not falling at sunset and sunrise, Aegis having default magic resistance 0 to everyone inside it, etc.
I’ll just note that the philosophy of House Jerbiton is that trying to compete over “who has the highest Art” is a sure path to a life of ugliness and deprivation. Jerbiton routinely simply stop improving their Arts once they can live a comfortable life surrounded by and patronizing beauty.
This doesn’t mean they wouldn’t want to be remembered for something! It just means they are choosy when it comes to such things. A mundane family, for example. The most recent generation want to retake Constantinople and lead the House to greatness!
Defeating a powerful mystical enemy the Order would definitely count as a legacy.
E.g. Pralix with the Spider. I'm sure the cabal that broke Tremere's bid for supremacy views theirs a legacy too.
Changing the focus of your House would count too. E.g. Quendalon with House Merinita.