It's definitely an interesting idea. You brought up Mage, and I can speak to that (more Awakening than Ascension but I can do some of that too since the diversity potential is similar in each).

In both versions of Mage (and all editions therein), build potential is... really vast. The games by default* have 9-10 spheres of magic (called Spheres in Ascension, Arcana in Awakening), which govern the different aspects of reality the mage can affect - time, the mind, elemental/physical forces, and so on. Each such magical sphere has different things it can affect at different levels - the specifics vary between games and even editions, but generally having a higher rating in a given Sphere-cana can let you do more. In Ascension, each dot comes with one to three effects that are the essence of that dot's capabilities, while Awakening's core books give sample spells outright. However, mages (and players) can create additional effects using these as guides. Further, they can combine effects from different types of magic to create something greater. There are common uses, like using Space/Correspondence to inflict an effect at a distance, but also true blendings, such as turning yourself into fire or even achieving some form of limited immortality. Thus, the build potential is limited only by imagination and the ST going "okay, that's probably too much" (and more the former...).

Play potential can be... less diverse than you'd expect given the freedom, but it depends. Both games have systems in place for using practiced or memorized effects rather than making one up on the fly, with the mechanics being more robust in Awakening. These effects may not vary mechanically from a suggested effect, but often have a benefit added for using the practiced trick - in Awakening 2e, Rotes allow you to add dots in a specified skill to the roll for the effect and gain extra "reach" to customize the spell with - more on that in a moment - while Praxes can get an exceptional success - and thus bonus effects - with fewer successes). Thus, there's benefits in using recurring spells, especially common buffs. However, the fact is that the spell suitable to a situation can change moment to moment, and thus there are plenty of times where you will pull a spell off the list that you haven't memorized, or that you come up with a unique effect. It also depends on the size of the group - a group with more mages will usually see the mages prefer to specialize in their Arcana and preferred styles of using powers, while a smaller group will likely see more generalists. However, even a low ranking in a single Arcanum can lead to a WIDE variety of spells, and spells tend to be fairly flexible, so even recasting the same spell can lead to different actual effects in a situation. And the fact that you CAN cast any of those spells means that while your play style may lead to a fairly stable set of effects, it's not like a Magic deck where every game with the same character will absolutely go in a similar fashion. Many buffs or precasts will be the same, but on the fly spells could change.

Awakening also has something called Reach - a mechanic that allows customization of a spell. Reach has several effects that are "generic" across spells - for example, using one reach allows you to take the casting of the spell from "ritual" length (minutes or hours) to casting in a round, change to the more advanced table for spell duration or area, or make it harder to dispel. In addition, most spells in 2e have benefits that can be purchased with Reach, adding or altering the effects. A mage gets free Reach based on the level of the spell compared to the level of mastery you have in the appropriate Arcanum (you have one plus the difference between your Arcanum and the level of the spell), but you can use as much reach as you want - but more Reach risks Paradox, the backlash of uncontrolled magic (your Reach exceeds your grasp - a very intentional turn of the phrase). This is how play can really vary - the same spell not only could target different people or objects, but be a very different spell. That said, the unique effects also trimmed down the spell list a LOT from 1e, so it's not necessarily a true expansion of the build potential, but it does make things very different..

*This doesn't get into Mage's alternate systems, such as lesser spheres or the Dark Ages "archaic sorcery" using pillars - this generally changes only the count of total "areas" of magic, though, and rearranges where effects go, and in the case of Archaic sorcery can limit the kinds of magic you are capable of but still leave a ton of potential.

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That said, there is something to be said for systems that are highly focused IF that is the intent. You can see this when you look at systems where a small set of "effects" is the only possible powerset normally available to a character, but they can twist those powers in useful ways. You see this in a lot of Brandon Sanderson's works, where characters sometimes only have a limited set of powers or even just ONE power, but become masters of what they have and do things well beyond. You can also see this with Iroh's philosophy in Avatar - firebending controls fire, but through studying the other styles, Iroh developed techniques that could expand the power of fire.