Update.
I finished the additional story for the third map and completed my first round of story editing, so the third map is pretty much finished. There are still a couple of minor things I want to get done (remove some superfluous stat structures), but other than that, things are looking good for map 3.
I've done more work on the custom abilities, sort of revamped them. Most of them now have mastery levels (basic, advanced, etc.) like the regular game skills. They work as follows:
+ a static amount of a stat (such as attack) and + a small percentage (where the percentage is of the game skill amount + the hero's experience level)
The static amount and percentage increase as the skill mastery increases. The skill mastery increases once the player has put a certain number of points into the skill.
I still have one or two more custom abilities to get hashed out, but they won't even be available to learn until the fourth or fifth map, so I still have time to get them done
I'm starting to run out of things to work on for maps 1 - 3, so it's about time to stop procrastinating and get to work on the story details for map 4

Almost halfway there!