Several things could be implemented like:

Custom Stylepack (Someone has to build that).
Custom villager skins (Someone has to design those).

The last one with "Might have to think that through completely" is mainly in reference to Minecraft's EULA, which states that we/us/you can't give players any advantage within game for money (which in turn includes patronage)

Implementing it in the JAR does NOT mean that you are violating the EULA.
How the server will implement it is the issue in reality. If a certain "feature' is ONLY made avail to "patreons" f.e. that would be a violation, but making it accessible to any player (at an extremely high cost of playtime/money earned/etc etc) as well complies w/EULA.
The code for implementing it and turning it on/off in config as a "feature" is awesome and does NOT violate the EULA.
It is a great feature that will bring more attention to servers that host this mod.

Simplest solution is expand the current config file option into something that intergrates into shop and server management plugins. Leaving the config option in place where patreons can be added/removed manually and/or automatically and updated when the mod updates. (short term) Later you would want a system that has patreon intergration that automatically requests that information keeping the config file as a backup (if possible)