I wasn't aware that Modular Abilities allowed for the purchasing of stunts...
The canon version may or may not, it's not totally clear. This version definitely does.
There really is only one Marked by Power
...
The primary issue I see with 'any sponsored magic' availability is one of roleplaying considerations related to actually receiving that power from that sponsor, and are thus avoidable by saying that the individual is capable of accessing 'self-sponsored magic' that produces essentially indistinguishable aptitudes.
Some of Sponsored Magic's effects are pretty intimately tied to its flavour. It doesn't refluff well.
I try to limit narrative as little as possible when I write crunch, as you know. But I really think that this idea is narratively unworkable.
Maybe I should rephrase this line of inquiry. What would have to be excluded from the list of options for an 'everythin not on this list is fair game' Modular Abilities power to be reasonably costable?
You'd have to exclude all Powers and Stunts that have not yet been written. Plus Limitation, because it doesn't do anything if you can switch it out like that.
Apart from that I don't think there's anything you'd absolutely have to exclude, but keeping Immunity and Refinement and maybe one or two other things would kick the Refresh cost up to something absurd.
You'd probably have to exclude Lawbreaker and most Items Of Power and most Sponsored Magics, too. But that's a flavour thing, so it's up to individual groups.
What if you made the traferrance time longer? It would force you be prepared ahead of time and would make it so you would have to really consider changing mid battle.
It would help, but the issues would still be there.