I can see why making the Mastery Abilities require pyramidal experience for improvement but I don't see why the second mastery should be harder than the first. I am inclined to give a special ability for every 5 EPs spent. What do you think?

(I know it's a special rule and the design may have been made pyrmidal for simplicity but Special Abilities is an added complication already.)

There are two considerations here to consider - game mechanics and "flavor". From a mechanics standpoint, you must consider that the Mastery level doesn't just define the number of abilities, but also applies as a modifier to die rolls in many cases, acting as an additional ability in the calculations. In that regard, it needs to be a pyramidal point system to reflect the increasing difficulty require to attain such mastery.

If you were willing to accept more bookkeeping you could argue that each Mastery Ability is learned and tracked separately, so you can but Fast Casting and Penetration both with a Score of 1 for 5 XP each, but this doesn't count as a Mastery Score of 2. Each would have to be raised individually.

From a flavor point of view, spell mastery isn't just a mechanical approach to improving upon a spell. Magi don't just "know" that if you want to fast cast, you have to do X, Y, and Z. Mastery reflects a wizards own, personal understanding of a spell and reflects how he went about gaining that particular aspect of mastery. To gain further aspects of mastery requires even greater depths of understanding, reflected in the increasing XP requirements. Although a Player simply chooses what Mastery Ability he wants, it isn't necessarily that easy for the magus. The magus studies the spell, applying XP towards it, and, in a flash of insight, develops a particular Mastery Ability. This is implied in the nature of books written on Spell Mastery which do not require a magus to take the same Mastery Abilties as the author.

I can certainly appreciate your arguments. I like your point that perhaps the mage does not get to choose what his insight is, although I would suggest that he works on the particular special he wants and so develops it.

I can also see in the games I play in that, at least up until now (i.e., ArM3 or ArM4 game), no one has really worried about mastery anyway so this may all be unnecessary. It was just something bugging me.

Well, to be more precise, I don't believe it should necessarily mean the magus doesn't know what mastery ability he is going to get. If a magus is trying to find a way to Fast Cast, then that is what he is devoting his time towards. What I meant to infer is that the actual practice of learning to Fast Cast (or do any other mastery ability) a spell isn't so ingrained into Hermetic Theory that the steps taken to do so are layed out for the magus. The Mastery Skill reflects a magus in-depth understanding of the spell formula. The "mastery ability" represents the tangible benefit he gains because of that level of understanding.

You could also think of it like this: say the magus develops Mastery 1 Fast Casting as of result of studying a spell. If he then wants to learn Penetration, it is harder, because he has to avoid making "changes" in his spellcasting that would affect his ability to Fast Cast. Thus, he is having to come up with a casting formula that will allow him to both Fast Cast and Penetrate. Now, you have to take this with a grain of salt, as it is modeling mechanics more than thought processes, but you could argue that once a magus begins the path toward mastery, he can't unlearn what he has already learned and is obligated to find ways to incorporate all previous mastery abilities with the newly sought one.

Tracking multiple mastery scores for each ability is a viable option, but in the long run the player is probably the worse for wear. Though it is easier to get lots of Score 1 abilities, it becomes much harder and more time consuming to get lots of Score 2 or Score 3 abilties.

I have reread your post and now have a better understanding of what you intended: to allow a new ability to be gained for every 5 XP spent, but the overal Mastery Score still only increases on the pyramid scale. This would allow the rapid acquisition of abilities, without overbalancing the effect that mastery has on spellcasting. I think that would work fine.

I initially thought you meant simply requiring 5 XP for each level of score (and thus each ability). That would be way out of whack.

When my group was on 4th Ed., no one bothered with spell mastery, either; but under the 5th Ed. rules spell mastery is much more attractive. Now they tend to master all of their combat spells.

The fact that aiming rolls are no longer needed for spells that can be magically resisted means that multi-casting Ball of Abysmal Flame and its ilk is much easier and risk free.

After one of my player's was hit by a rival's Pilum of Fire (although he was only singed for 1 Light wound, I believe) he has added Resistance to his Mastery skills so that doesn't happen again.