Well, in principle a Breakthrough allowing the creation of longevity rituals that don't cause Warping. But, I'd suggest doing the opposite: instigating a political change in the Order that will allow it to accept members with Might, i.e. immortal wizards. And concurrently a introduce way or ways to achieve said immortality, so that the standard trajectory for magi would be to become some sort of Immortal magus.
This has the advantage that growth through Experience is effectively capped; otherwise, you'll get some crazily-over-experienced magi around. And I think it's more colorful. It would be a pretty massive change to the current system, however.
Concurrently with it, I'd advise a plotline of "Diminishing Magic" where the number of Gifted children drops, so as to keep the total number of magi manageable.
One option to do it is just to allow a Breakthrough that allows magi to become a Magic Being upon entering Final Twilight. Perhaps discovered by a Criamon or Bonisagus magus, and spread throughout the Order. This is the simplest way.
I'd actually prefer to have a variety of Immortal types, by having multiple ways of becoming immortal spread throughout the Order. Some through House Cults, others through Order-wide Mystery Cults. Some examples:
- As a minor change to House Bjornaer, allow their Great Beasts to be their Inner Heartbeasts, but with a Fast Acclimation flaw so that they tend to stay in the Magic Realm.
- House Criamon already has a variety of ways of becoming Immortal. Utilize them, especially the Tangential Magi.
- House Merinita already has immortal magi through (Faerie) Becoming; let them be known.
- Let Hermetic Alchemy spread through House Verditius, and perhaps beyond. I'd perhaps amend it to transform the magus into a magic item rather than purified flesh and blood.
- Let House Tremere develop a House Mystery Cult revolving around spirit-magic, including the Living Ghost.
- Let the Tytalos Titanoi become servants of the Titans, a la Votaries.
- Let these, or similar, Mystery Cults generally spread throughout the Order. So you could have the Order of the Green Cockerel (perhaps separately from House Vertidius) allowing magi to reach alchemical immortality, a large surge in the Titanoi as more and more magi become subservient to the old Titans, an astrological cult which teaches worship of the stars and apotheosis as a new star (Ascendancy to become a Daimon), and so on.
That's a lot more work, but more interesting, IMO.
Finally, consider what Science is in your setting. You could decide to do a Magical Revolution instead of a Scientific one; I'd do that, personally, as combining magic with science leads to one big conceptual mess, and as a lot of technology could be implemented magically rather than through science. If you do do that, consider whether you even NEED to reach modern times. The "Magical Revolution" could develop quickly, so that you'd reach current-levels of magic-tech within the magi's standard lifetimes. Do you really NEED to go to the 21st century? Wouldn't just reaching the 15th century, after a fully-developed Magical Revolution, suffice?