Not too long ago I had the idea for Altered totems for Shamans and for Arch-Mage aspects for Hermetics.

Just recently I was thinking, with judicious assistance from the peace pipe.

Where are the initiate only spells? aka You need to be a 3rd grade initiate to even look at that spell formula without it twisting your mind apart.

Or maybe you need a magic rating of X and metamagic B to be able to cast spell Y at all regardless of force.

When you initiate you gain a Magic point and a metamagic.

Where are the spells that only initiates can cast? What would be a few good examples? Is it possible for a spell to depend on metamagic to be cast?
Examples that comes to mind.
Sphere of Minor Invulnerbility : Spell shield and physical barrier requirement - shielding metamagic
Deflection : Complete deflection of spell to new target in your LOS. Needs to be cast at force of incoming spell, with same TN and number of successes. Drain +1 to original spell's drain Requirement : Reflecting metamagic and Astral Perception
(this one probably ends up being more of a metamagic itself)

Maybe I'm just thinking too much along Prisons & Giant Lizards (D&D) where wizards or druids would gain stronger spells that they couldn't cast before.

The way Shadowrun handles it, anything that required an initiate or another metamagic technique to perform is a metamagic technique of its own. They are the "uber spells" in most cases. Your "deflection" spell is almost exactly what the Reflection metamagic is, for instance.

Doc's got most of it. The other benefit is that with a higher magic, you don't take physical drain from high-force spells.

Let's say that by some miracle, two characters learned a force-20 fireball. One's a just-starting character who spent her first 20 karma on it; the second is a grade 9 initiate with a force-5 power focus. Both of them somehow made a set of amazing rolls, and managed to learn the spell.

Starting Sally is looking at an 11S physical, trying to cast the spell at max force and at Light damage. If she tries for a base damage of 20D, she's going to be taking an 15D physical, which is going to likely kill her outright.

Experienced Edgar is much better off. While he's still going to be dealing with the 11S drain, it's only Stun for him. Even if he maxes it out to a 15D, the worst that will happen is that he's knocked out. With his added dice from Centering, he might even be able to lower it a bit; and he's going to have a lot more spell pool to help witht he drain.

So, Edgar is clearly much better off at the high-powered spell. Sally can cast it, but only at risk of frying herself by channeling too much mana through her body. Edgar, being a powerful initiate, is much more experienced and capable. He can throw the spells that would kill Sally outright-- sometimes with a big headache involved, but he can still cast them and expect to live.