I think for most people, the most useful thing would be storing fonts on an SD card and selectively loading whatever is needed. I say most useful because that is the most common large storage device that people pair with these sort of displays, for 24-bit images and so forth.
In the end I think the fonts aren’t too crazy in size that I can’t deal with them. I know that my total sketch size at the moment is 83kb out of 262kb or so that are available, and I imagine my non-font sketch size is no more than 15-30kb. But i wouldn’t have the space to not selectively ignore characters I don’t need in my fonts. For that reason it would be convenient if the mfGFX library had a way to include fonts where I didn’t have to have a continuous series of characters. Meaning, rather than having to include 0-9, if I only needed 0, 3, 4, and 9, only including those characters. As I understand it right now I have to include a full set [or else make 3 fonts, one for 0, one for 3-4, and one for 9]