The single source of truth meta rule does not mean that homebrew content is not allowed!
The opposite is true, players are encouraged to design and propose—or copy "inspiration" from elsewhere, adjust, and propose—their own feats, spells, classes, and magic items.
Since the setting strongly diverges from the 'default' Pathfinder setting, feel free to consult with your GM on the availability and setting-appropriateness of concepts.
Some guidelines have been written out to assist both when adjusting existing content or designing new things:
Conversion
The conversion of spells, feats, classes, and other rule-related things for the Maalum setting will mostly be done in a 'see what players are interested in, and convert that' fashion.
Since there is a lot of content that could be ported, the GM will adhere to this 'player driven design' plan so much so that only things the players are interested in are likely to end up on the Rules wiki.
This means that some initiative lies with the players to prod the GM with questions and suggestions of things they would like to see available.
Inspiration
Note that the unused parts of the Source Books (classes, spells, feats, and any mechanic that is not used) can offer great inspiration for the rules-wise implementation homebrew spells, feats, classes, and other custom things.
The same holds true for other books. In particular, the following source books are of great use for their class feature alternatives described in archetypes and classes:
- Advanced Players Guide
- Advanced Class Guide