Or a slight variant:
Wish is not a spell you can learn. It is treasure you are awarded. Just like a +5 vorpal sword is not a thing you can forge. It is something you are awarded.
I must say that I like the idea of rare spells as treasure although (ironically, given the thread title) I would use it more as a mechanism to re-introduce some spells that were deemed too problematic, possibly due to balance or abuse issues, to update into 4E.
As for re-balancing magic at higher levels, I was working on a magic system inspired by the Book of Nine Swords before 4E was announced and (IMO) made it unnecessary. Some of the changes that I thought of making were as follows:
Spell Slots
Spellcasters get fewer spell slots than they do presently - the working assumption is 2 + 1/2 caster level, so a 1st-level spellcaster gets 2 slots, a 2nd-level spellcaster gets 3 slots, and so on. A spell slot can hold a spell of any level, but each spell slot must be filled with a unique spell (no duplicates). Between encounters, a spellcaster can fill an empty spell slot by meditating for 2 minutes. During an encounter, a spellcaster can re-fill an expended spell slot by spending a full-round action that attracts attacks of opportunity, and succeeding at a Concentration check (DC 15 + 2 x spell level). A spell that has a duration continues to occupy its spell slot until its duration expires, or it is dismissed or dispelled. This effectively sets a limit on the number of active spells that a spellcaster can maintain at any one time.
Spells
Because spell slots can hold spells of any level, the main limitation on a spellcaster is the number of spells that he knows at each level (this is essentially the approach used in the Book of Nine Swords). The working assumption is that a spellcaster knows two spells at 1st level, and learns an additional spell at each level thereafter. Knowledge of spells is already effectively limited by caster level (you need to be a 3rd-level spellcaster to know 2nd-level spells, for example). Some spells (in particular, the more powerful ones) have additional prerequisites, such as knowledge of a minimum number of related spells, or knowledge of a specific spell.
One concept adapted from the psionics system is that a spell (even a 1st-level spell) should be useful at all levels. Hence, there will not be any spell level-based damage caps, and certain spells may have varying (but essentially similar) effects at different caster levels. In addition, saving throw DCs are based on caster level, and not spell level (the working assumption is 10 + 1/2 caster level + key ability modifier).
An attempt will be made to remove the concept of spell level entirely, since there is no need for spell level when assigning spells to spell slots and when setting saving throw DCs. For purposes of learning spells, a minimum caster level can be listed as a prerequisite.
Fundamentals
Certain spells (most likely, low-level spells without prerequisites apart from minimum caster level) also grant knowledge of fundamentals (the term is taken from a similar concept for the Shadowcaster in Tome of Magic). A fundamental is a minor magical effect that a spellcaster can use at will. For example, most basic offensive spells will have fundamentals that can be used to launch a magical attack against an opponent. A spellcaster that runs out of spell slots in the middle of an encounter can thus either attempt to re-fill an expended spell slot, or fall back on his fundamentals.
Swift Actions
In the same way that a standard action can be traded for a move action, a standard or move action can be traded for a swift action. Spells must be balanced around the idea that a spellcaster can cast up to three spells in a round.
Metamagic Feats
Metamagic feats should be usable once per encounter and should be balanced differently since the concept of spell levels may be removed. Empower Spell and Maximize Spell are tentatively balanced by the need to spend four and six swift actions respectively before they can be used on a spell (this has the added side-effect of ensuring that these feats do not come into play for a few rounds, and may also delay the spellcaster's use of his best spell till that time). Quicken tentatively reduces the caster level of a spell by six (so that a 7th-level spellcaster casting a Quickened spell casts it as a 1st-level spellcaster).
Specialization Benefits
To be frank, I've never really liked the way that the rules handle school specialization. In my view, they are at the same time too restrictive (you can't ever learn spells from your prohibited schools) and too lenient (apart from your one bonus spell per spell level, you can prepare any spell in your spell book; apart from the one spell of your school you must learn at each level, you can scribe any spell from any non-prohibited school into your spellbook). Because wizards tend to choose the best spells in their non-prohibited schools, the most distinctive thing about a specialist is what schools he gives up. Assuming they gave up the same schools, an evocation specialist and an abjuration specialist may end up with very similar spells known and prepared. It seems rather perverse to me that a specialist is better defined by what he doesn't know than by what he does.
Given that a spellcaster will know spells equal to his level +1, a specialist can be defined by (and specialization benefits can be given out on the basis of) the number of spells of a particular type that he knows. Specialization benefits can start at knowledge of 6 spells (about a third of a 20th-level spellcaster's entire repertoire), and additional benefits granted at 12 spells (about half) and 18 (almost all), and can increase based on the number of spells known of a similar type. Spellcasters that learn a variety of spells gain versatility, but lose out on or delay their specialization benefits.
