Magus Arcana - Metamagic

[Irontruth]

It feels to me like the low level options are too restricted by their lack of progressive uses per day, but I wouldn't want to increase the number of uses for higher level abilities.

My simplest solution is to reorganize the abilities under one ability, Metamagic Arcana (can come up with a better name later). It can be taken multiple times, each time you select it, you gain a new metamagic feat that can be applied. The level restrictions for each would be the same (though I think I would increase Empowered to 9th level). Every time you take this ability, all previous Metamagic Arcana selections gain one additional use per day.

If a character took nothing but this ability (and adjusted Empowered), at 15th level they would have:

Still Spell 5/day
Silent Spell 4/day (those could be switched)
Empowered 3/day
Maximized 2/day
Quickened 1/day

They could hold off Still or Silent until 18th level to gain an extra use of the 9 level or higher abilities, but then you're deciding your 18th level ability is Silent Spell 1/day.

[Freesword]

Actually, I have another proposal that is even simpler.

Remove the /day limit entirely and instead make it a swift action expending an additional spell slot of the metamagic adjustment or higher so it ends up costing you 2 spells to apply metamagic on the fly.

Still and Silent - 1st level or higher
Empowered - 2nd level or higher
Maximized - 3rd level or higher
Quickened - 4th level or higher

The level prerequisites for each arcana stay the same.

As an example for clarity:

Fireball (3rd level) spell. Empowered using the empowered magic arcana.
Expend the prepared fireball (3rd level slot) + a spell 2nd level or higher to pay for the empower.