I've been thinking about the magic system in the World. Lest I forget, here's what we have: Spells come in three "levels," indexed from zero.
Zero-level spells are utility spells, cost only unflavored mana, and can be learned directly from unguided research or when a mage attunes themselves to a nexus. (Aside: Nexi litter the map, and come in several 'flavors'. A mage can bind themselves to one nexus for each day that the current telling has run, and it grants them a new zero-level spell, and a new flavored essence of the same type as the nexus.)
First-level spells can be more powerful utility spells, as well as basic combat spells. First-level spells can be learned by preforming research guided by two zero-level spells, or a zero-level spell and a first-level spell. This research cannot be successfully completed without five bound nexi. First-level spells are drastically more powerful than zero-level spells, which is why they are limited by costing flavored mana, in addition to unflavored mana.
Second-level spells are extremely powerful utility spells, or more commonly, advanced combat spells. Second-level spells are learned by preforming research guided by two first-level spells, or a first-level spell and a second-level spell. This research cannot be successfully completed without fifteen bound nexi. Second-level spells are even more powerful than first-level spells, and thus cost many flavored mana, sometimes in multiple varieties for a single spell, in addition to unflavored mana. Some second-level spells also grant a free zero-level spell when learned that is a lesser version of themselves that can be used far more frequently, via unflavored mana.
Mana comes in a small number of flavors, which have yet to be determined. Each flavor of mana is associated with several themes, and those themes imply mechanics. If a spell costs generic mana, you may use flavored mana to pay that cost, but you cannot use unflavored mana to pay a flavored cost. All players have a small pool of unflavored mana that increases by some means (as yet, undecided) as an island progresses, and refills fairly quickly after use. Mages also have up to X other pools of mana, one of each nexus that they have attuned themselves to, which are usually dormant. A mage can choose to tap one of their flavored pools (a free action in combat), at which point they gain access to a slow trickle of flavored mana for a short time until the pool is depleted, at which point they will have no access to that pool again until the pool has had time to recover (X minutes outside of combat to refresh all flavored pools).
When a server comes online, the game marks X% of the possible spells of each spell level as "learnable" on that telling. From the list of unlearnable first-level spells, each player receives a random "innate" spell. This spell functions normally, except that it can be cast with unflavored mana instead of its usual flavor. Players are not told what their innate spell is until they have a large enough unflavored mana pool to cast it.
For some spell ideas I have, see: http://espoires.blogspot.com/2011/01/spell-ideas.html
Update: All players have an "essence" stat, which determines the rate and cap of mana generation. A player's true maximum is 100 total mana, (important when considering the effect of mana potions), but if their essence is 5, (the base amount), then they will regain mana fairly slowly, and only generate up to 5 mana before their mana generation halts. Any mana held beyond the character's essence will slowly decay.
Essence can be boosted in a number of ways. Potions can give a 10-minute essence boost, well-made staves, wands, robes, shields and jewelry may boost essence while worn. There exist multiple ways to permanently boost a character's base Essence, once of which is being a subscriber, which would boost Essence by 100% or 20, whichever is smaller at the time. Being a combat mage would be balanced around having access to the subscriber boost, thus if you try to be one without it, you would be slightly less effective than a melee or archer (which may be worth it if you need the variety in your group).
When a player taps a flavored mana pool, it would convert 5 of this basic essence into 5 flavored essence for a limited time. In a long battle, a mage might "burn out" by expending all their access to flavored mana. This is intended to be similar to a physical fighter becoming fatigued. If a mage does not gain any flavored mana for 2 turns in combat, a random completely empty pool will become partially refilled, allowing to to be tapped to grant 1 flavored mana immediately.
Availability of flavored Nexi: Players should have no problem binding themselves to their maximum number of Nexi, but they also should have difficulty gaining access to that many Nexi that are all of the same flavor. Thus, they will need to choose which flavors to go for based on what has been discovered and whether those are guarded by hostile monsters or not.