Modular Powers is 20 points for Modular Powers 1, + 10 points/level. This advantage holds points of Alternative Abilities for a main ability bought at its full cost. For example, if you have Flight  as your main ability, an Alternative Ability would cost up to 8 points. So, Modular Powers 8  would cover the cost of any Alternative Ability to Flight.
Modular Power Talent is 10 points/level. This is the power Talent for Modular Powers. Modular Power Talent adds to every roll made to use the main ability or any of its Alternative Abilities or to use any of those abilities well.
Modular Powers and Modular Power Talent are can both be modified for Limited Scope and the Cosmic Power Modifier, but only Modular Powers can be modified for Power Modifiers other than Cosmic. For Limited Scope, you could use the version of the limitation on Sorcery or I would personally recommend using the version from Ritual Path Specialists from Pyramid #3/66: The Laws of Magic as a basis as it is much more nuanced (though, I would only allow it down to a certain level or you’re just better off buying a non-flexible set of powers). The unmodified scope is enough to cover anything the GM agrees could be used as a power in the setting. For example, an Alternative Ability to Flight in the above example would not be limited to abilities related to Flight. Instead, Flight could be thought of as a defining ability for that character out of a pool of any possible power. (In fact, now that I think about it as I’m typing this, I’d probably allow a +1 to that one ability for free or at least have that as an option for a perk—call it Signature Power.) If the abilities should only relate to Flight, then something like Limited Scope, Only for abilities directly related to flying (-50%) should be applied to Modular Powers and Modular Power Talent.
Furthermore, the main ability can change as part of advancement. As long as you enough levels of Modular Powers to cover the cost of your old main ability as an Alternative Ability, you can put those points toward the cost of the new main ability. For example, if you have Modular Powers 8  and Flight , you can reallocate all 40 points for Flight to your new main ability because you are able to cover its cost as an Alternative Ability. If you do not have enough levels of Modular Powers, you may only reallocate the points that can be covered by your level of Modular Powers, but you may still use the old main ability as an Alternative Ability. For example, if you only had Modular Powers 4 , you would only be able to reallocate 20 points of Flight as you can only cover 20 points as an Alternative Ability, but you may continue to use Flight.
Optionally, for simplicity, you could drop the idea of a main ability altogether and add Modular Power Pool at 5 points/level and require characters to have the same level of Modular Powers and Modular Power Pool at the same level. The only reason I didn’t bake the cost of Modular Power Pool into the cost of Modular Powers for this option is because Modular Power Pool shouldn’t take any modifiers as it’s really just a nameless main ability.
I would use this system as flexibly as one would use Wildcard Powers from GURPS Supers. For the above example using Flight, I would be willing, as the GM, to guesstimate what feels like a 40-point ability rather than calculate new abilities during the game. This kind of flexibility is the very reason I sought to create a powers/magic system like this one.
And that’s it. It seems simple, but it took me a lot of tinkering to get all the way back around to what amounts to an alternate version of the system from GURPS Thaumatology: Sorcery.
GURPS Thaumatology introduced me to the idea of ‘magic as powers’ or ‘powers as magic’, which is a nickname for an advantage-based power system. Since I first discovered this idea, I have always preferred powers and magic as powers to the system laid out in GURPS Magic because it made it easier for me to easily create new spells, which is something I really wanted. As I discovered other flexible narrative-focused RPGs like FATE, I wanted to bring that flexibility to GURPS. As a GM, I didn’t want to spend a bunch of time creating spell after spell, I wanted to be able to say, yeah, that sounds like it’s worth about 50 points and you have the ability to cast 50-point spells, so it works. Well, that brought me to Wildcard Powers in GURPS Supers as well as Using Abilities at Default in GURPS Powers. (It was also heavily inspired by the flavor of GURPS Powers: Divine Favor, which allows a character with Divine Favor to call upon their deity for a prayer/power, but that works in quite a different way.) The idea of Using Abilities at Default and improvisation became a core part of the magic as powers system of GURPS Thaumatology: Sorcery, which I really loved. Still, that flexibility still paled in comparison to what I could do in FATE and what Wildcard Powers allowed.
My first thought was to create a flexible magic system that allowed one to use an ability at default based on the power Talent itself. Of course, there was already a thread about that very idea. Feelings were mixed, and it didn’t end up feeling like the solution I wanted.
So, I started thinking about Wildcard Powers again. I realized an inherent issue (from my perspective) of Wildcard Powers (and, by extension, Using Abilities at Default), which was that Flight!  would be way less flexible than Control Time! 1 ; for example, I could get Flight out Control Time! 1 but not Control Time 1 out of Flight!. So, I started a thread about it, and it came down to the idea of Wildcard Powers not being very realistic for a reason—I mean, Superman was able to fly fast enough to turn back time; so, Flight!  should be way more flexible than I am giving it credit for. That’s an extremely fair point. Though, I later realized that the flexibility is probably more related to the power Talent than the ability itself. I’d find it easier to justify a more flexible scope for Flight!  if I had the Kryptonian Talent versus just a plain-old Flight Talent.
Well, Christopher Rice has a blog post (Call of the Wild) about assigning different multipliers to Wildcard Skills based on their breadth, so I figured I could do the same to Wildcard Powers. It’s also important to note that the base multiplier for Wildcard Powers is ×4 instead of the base ×3 for Wildcard Skills because Wildcard Powers allow/require one to use the base power while using the improvised power. (I was starting to come up with a very similar system to the one found in this thread.) So, I tried to figure out how to tie the cost/breadth of the power Talent to the multiplier for Wildcard Powers.
But I was already hatching a different idea. A Modular Ability is expensive because it can hold a certain number of flexible points. What if I extended that to include Alternative Abilities? So, if I have Example Spell  and Sorcery 2  and paid full-price for both, could I just use my Sorcery 2 to improvise any 10-point spell as a 2-point Alternative Ability? That seemed like it would probably be very broken, but the math lines up with Wildcard Powers. Example Spell  plus Sorcery 2  (ignore that I took out Sorcery 0  for now), is 30 points, which is the cost of Example Spell ×3, which is the base multiplier for Wildcards. Of course, Wildcard Powers uses ×4, but, again, that allows/requires the use of the base power with the improvised power. Now, that pesky Sorcery 0  was starting to look like power Talent 0 because the base cost of a Modular Ability reflects the breadth of abilities that it covers, and I had already devised that the breadth of the power Talent should theoretically determine the breadth of Wildcard Powers or Using Abilities at Default. This pretty much lined up with Modular Abilities (Cosmic Slotted Power), which has a base cost of 7 points. To make it equivalent to Sorcery, I modified it by Limited, Advantages Only (-10%), Physical (+100%), and Reduced Time 1 (+20%) turns it into 14.7 points, which rounds up to 15 points, which just so happens to be the cost of Cosmic Talent. Coincidence?
Probably I think not! Sticking Magical (-10%) back in there turns it into 14 points, which is still close, but I wanted to leave out a Power Modifier for the base build.
So, I called it Modular Powers and Modular Power Talent.
Modular Powers is 10 + 10/level points just like Sorcery, and I would be more than fine with someone buying Modular Powers 0.
Modular Power Talent is 10 points/level. It can also be modified for Limited Scope, but it is not modified for Power Modifier unless it is the Cosmic Power Modifier (which explains why Cosmic Power Talent is 15 points/level).
I could have gone either way with Limited Scope on the Modular Powers advantage itself. Modular Abilities don’t tend to get a big reduction for Limited Scope, but Sorcery sets a weird precedent. If I didn’t want the Modular Abilities portion to get limited or enhanced (except for by relevant Power Modifiers), I would turn Modular Powers 0 into Modular Power Talent 0 because that zeroth level should be modified as the base cost of Modular Abilities is dependent on scope.
Even if it’s not perfect or RAW or whatever, all new rules start somewhere, and I think the math ended up being quite fair. Really, I just recreated Sorcery with some extra rules and flavor—you can apply the same idea to Sorcery as I did above.