Triple creature MR against Hermetic magic. Significantly lowers magi power levels, of course.
Spell magnitude is x3, instead of x5. The lower spell levels mean increased magi power levels. Can be combined with Arts as Abilities to make the game more consistent, more difficult to run (multiplying by 5 is easier!), and keep the power level about the same as in the core rules.
All magical Size increases are +1 Size Score (i.e. +3 is x10 mass) per magnitude. This includes affecting large Targets, increasing the Size of a creature, or so on. This also lowers magi's power level, preventing them from affecting the whole kingdom or so on.
No Virtues & Flaws limitations (aside from balancing them, and common sense). Expands character-creation options considerably, which can result in greater power.
The Limit of the Eternal: spirits cannot be killed. Those drained of their Might Score will slowly recover it in the right Supernatural Aura, at a rate comparable to wound recovery. This rule applies principally to demons, ghosts, and so on, and means these things will usually be bound or exorcised rather than killed. The best way to deal with a demon is to might-drain it, bind it, and bury it in holy ground / the dungeon under your covenant's Magical Aura, or so on. This rule somewhat lowers magi power level.
The Limit of the Soul: No Mentem Art. Aka "Free Will b**ches!" and "Murder mysteries for the win!". Some Mentem guidelines may still be accessible through other Arts (exorcising a ghost = vim, inducing lust = corpus, and so on). Obviously, greatly lowers power levels.
Difficult Imaginem: double (triple?) all Imaginem spell guidelines. Because it's too easy (especially affecting one sense, e.g. vision; invisibility is especially troubling).
You may apply Ceremonial Casting to Formulaic spells. Increases magi's power levels, though not by much.
Gain 30 XP per year, every year [30 XP is for magus/Mythic Companion; Companions/Grogs get 20 XP for Wealthy, 15 for normal, 10 XP for Poor]. Seasonal activities determine only the limits of what you can invest in; you can always invest 5 XP in any plausible Ability, but higher investments require a Seasonal Activity such as reading a book (the Quality becomes the XP you may invest). Increasing Arts always requires a dedicated activity (learning from vis/book). Each laboratory activity wastes 5 XP (so 4 seasons wastes 20 XP, leaving 10 XP gained in things like Magic Theory; RAW will give you 8 XP for such a year). This rule is slightly complicated, but Huge in its way without greatly affecting anything else. It would usually lower magi power levels, I suspect, as in-game characters will often gain more than 30 XP/year - but that's saga-dependent.