Quote Originally Posted by Klara Meison View Post
Well, suppose you have two people (Jeb and Bob) who want to write some additional stuff as modifications to D20L core. One (Jeb) wants to write a book on dance fighting, and another (Bob) wants to go into Star Wars space fantasy stuff. If you don't think about possible mod support while writing the core rulebook, you may end up in a situation where both of these writers find it very hard to make any changes/additions without seriously breaking things. For example, suppose that Jeb wanted to write a dance-fighting spellcasting class. Jeb wants to give this class a special spell list, tailored towards dancing. So he comes up with some new spells, new mechanics(how to start dance-offs, how they work, how they function with core spells), and gives that class some core spells at weird spell levels, like Freedom of M. at lv 3, balancing other class features/spells around that to insure it's not gamebreaking at those levels. Well, in Pathfinder, this would cause serious problems. For starters, due to ripple effects caused by magic creation rules this change should drop the price of Rings of Freedom of Movement significantly, affecting medium- and high-level combat all over the system. So if you want to allow people to create classes with different spell lists, you may need to think about fixing this issue(among others).
Fortunately, this was one of the earliest issues patched in D20 Legends. Spells do not vary in terms of spell level on spell lists. They're always the same level of spell. If a class has access to a spell earlier than usual, it's acquired as a class feature (which you spend talents on), and will detail the special circumstances of how you get to use the spell. The ability might let you cast it using a lower level spell slot than the spell normally requires, or might grant it as a SLA, etc). This way some classes can acquire certain thematic spells earlier but it never affects the prices of magic items. Likewise, since save DCs aren't tied to spell level in D20-L, it simultaneously means having a higher level spell won't be useless or innately overpowered (in traditional D&D, 3/4 and 1/2 casters have terrible save DCs, meanwhile being able to cast an 8th level spell at 9th level as a SLA has a major impact in terms of save DCs, but you'll see stuff like this on monsters in regular D20).

Now suppose Bob releases his modbook(which has lightsabers and Force), and a table somewhere decides to try employing both at once. If that possibility is something you want in your system, then you may want to write some rules on the order in which modifications should apply, such as "First books that modify major mechanics, then books that modify minor mechanics, then books that modify specific classes, then books that modify specific class features", so as to insure that contradictions don't arise. Likewise, if you yourself plan to release supplemental books that modify the core system in some way without being patches, such a "load-order" might be very important. You may be familiar with the concept from Skyrim mods.
One thing I wanted to establish is the concept of Core First, Else Second. For example, if you've got two rules that contradict each other, core takes priority unless a group is explicitly using the other rule. Here's an example: let's say you're trying to decide something concerning Alignment for your character. The Books of Vile Edginess or Exalted Hypocrisy would be optional adjustments, but their publication doesn't change the core functions of Alignment unless your group has explicitly chosen to use those options. Similarly, if you were discussing alignment questions with people online, the aforementioned books wouldn't enter into the equation unless it was a question that concerned them specifically (not that people wouldn't bring them up anyway, but it's a lot easier to dissuade such notions with a clearer book priority).

Now that you mention it, perhaps coming up with a more official priority list might be an option.
I'll have to give it some thought. Moddability is key for D20 games, which is a big part of why I've been making it so modular from its conception.

My fence of text may seem a bit all over the place, so here is my main point: if you don't design the system to handle user and/or other author modifications from the start, it's going to be a terrible mess when someone does try to modify it.
One thing I intend to do is scatter little text boxes around the book explaining certain things about the game, kind of like little strategy guides, suggestions, and behind the scenes considerations and/or explanations for why certain things are done the way they are. These notes will hopefully help people get accustomed to the nuances of the system faster and make it easier in general to mod things, since players and GMs will have an easier time understanding not only how things work but why.