However I personally believe that a tilemap plugin with character mapping capabilities should come first. The current spritefont pluging kind of does that, through a string, and aligning Nth character with Nth tile (based on tile size and image zie). The characters in the string are all unicode so you can assign any letter to anything (as long as you make the tile).
I'm not trying to usurp the thread for tilemaps entirely, but static-width spritefonts are just custom tilemaps in their own right and have a larger overarching reason to be implimented (thus more reason to impliment spritefont tile assignment)
Spritefonts can be artistically modified more than a regular font/webfont could be. Since font creation tools have no central Game-Maker hub for people who would need to make one, it's easier to make your own graphical font via illustrator or photoshop as you can guarantee the outcome rather than looking through 1000 fonts and still being confused and unable to find out how to make one of your own. (Font making is pretty niche, as most people don't give it much thought)
However if you're going to need a basic font, not a graphically enhanced on, I do recommend FontStruct and Font2Web (Fontsquirrel still has issues with FontStruct). As with creating any font, you would have to create support for whatever unicode characters you will use (All fonts don't have all characters so even random webfonts may be incomplete, all the more reason to make your own webfont/spritefont)