So I was skimming through the main rulebook, as I sometimes do, and some thoughts occurred to me. Parma Magica (and Aegis of the Hearth) presents some problems for me. So I thought I'd throw out some ideas and see what people think.
First, Parma and the Aegis of the Hearth don't cause Warping. This is basically at odds with every other magical effect in the game. The Aegis could easily be hand-waved if area wards didn't cause warping to occupants as well (something I'd like to see happen), but magi are basically living under a magical shield all the time and don't take warping from it. Additionally, when you extend your parma over someone else, they don't take warping either (AFAIK). Now this is a simple game-based reason so that players can live with a Parma all the time, but that doesn't make me like it.
Second, having played multiple sagas, there's always what I like to call the 'Parma Rush'. Players study Parma Magica hard early game until they're basically immune to the effects of Might 20-25 creatures. It's easy, it's pretty quick - if you have books, but Parma is supposed to be a big secret, and since it could be learned by any Gifted person, books existing are a big problem. This annoys me as a game master, because I'd rather players do neat things and I'd like to be able to employ lower might creatures early in a saga, but boosting Parma is so very effective you're crazy to not do it. Parma is still a time/XP sink and both are very valuable to magi.
So let's move on to my ideas for a solution. Magic Resistance arises from the Gift, just as creatures with Might get Magic Resistance. It's something that's inherent to being Gifted, and to a lesser extent, granted by scores in other supernatural abilities (this explains the lesser magic resistances found in hedge magi). Normal hermetic Arts scores already grant Magic Resistance.
So let's make Parma Magica just an improved form of 'natural' expression of the magi's Gift. This explains why any Gifted person can learn it, and any unGifted cannot, even though it's an Arcane Ability. Bonisagus figured out how to better 'tune' his Gift to provide Magic Resistance and, importantly, shielding himself from the social effect of the Gift. This would also explain why it doesn't cause Warping, as it's not a power or spell any more than the Gift is a power or spell. Knowledge of the Parma would also allow you to temporarily attune the Gift of others to have the same effect (thus you could activate a Parma on a Gifted apprentice). This would disallow using Parma to shield unGifted companions, however, as they have no Gift, though we could allow spells that provide Magic Resistance once again (which would cause Warping per the normal rules). It would also increase the social isolation of magi a bit (no covering mundanes with Parma to lower the creep-factor), so maybe increasing the rate at which mundane folk acclimatize to magi might be warranted.
Now, to remove the 'XP sink' nature of the Parma Magica, let's make the Parma score equal to the magi's Warping score. This provides young magi with a terrible decision; quickly gain Warping to improve their Magic Resistance, but in so doing shorten their Hermetic lifespan. Or they could actually study Arts and improve their Form scores, which I view as the preferable alternative. This keeps younger magi at much lower magic resistances outside their chosen Arts. Old, warped magi always have mighty Magic Resistance - this was always the case - and magi will always accumulate Warping, so the increase of Parma can be viewed as an upside to the 'punishment' of an increase in Warping score. And as mentioned above, if we allow spell-based Magic Resistance, magi could use such spells on a short term basis to shore up their defenses (probably not stacking with Parma or Form scores). Because there is no 'studying' Parma, there are no books about it, and that particular problem goes away.
Now one problem I foresee with this is that the power-gamer will always just boost his Warping up to 5-6, on the reasoning that the saga won't last long enough for it to be a real problem for him. I'm not certain of a solution for that, though IIRC some magical traditions have problems every time their Warping score increases, so if magi only underwent Twilight when their score increased this might serve as a deterrent. This would have the happy side effect of making the "Give someone 2 Warping Points" spell less effective as a "Instantly force a twilight check" spell.
As for Aegis of the Hearth, as I mentioned earlier I'd like to see area-based Wards only cause Warping to the area, not its inhabitants, but I can also see the Aegis as a tuning of a Magic Aura to naturally produce the effects listed, much as other realms can tune auras to produce certain effects. This might limit an Aegis' power based on the power of the Magic Aura it's cast in, and I don't see this as a bad thing necessarily (old, magically strong covenants have more powerful Aegises already).