I think it'd be good to have a
comprehensive curated list of fan-made mechanics; I'm going to use this post as sort of a Table of Contents and attempt to map some sort of ontology/categorization onto them. The categorization isn't going to be exclusive - if something uses a mat and counters, it'll be in both lists, etc.
This thread should be considered a perpetual work in progress - if you have a new mechanic, or find one I missed, reply here or DM me and I'll get it included at my earliest convenience obviously like a wiki would be better for this but a dominion fan card wiki is beyond the scope of my wallet and/or attention span, so it's a threadInvolving Counters/TracksThis is anything involving a place for coin tokens, or anything involving a separate track that something moves on
Involving Mats & Coin tokensThis is anything involving mats like the official ones for Exile, Coffers, Tavern Mat, etc.
Involving Other Card ComponentsNew landscapes, new things with a side deck a la Boons or Hexes, or things that add a new supply pile like Looters or Young Witch, or things that add a new non-supply pile like Spoils etc.
Involving Other Non-card ComponentsThis is anything involving other components (boards, dice, etc)
Mechanics that change over the course of the gameThis is mechanics that explicitly change over the course of the game - official cards that would belong here are Pirate Ship, Miser, Travellers, etc - strategy related changes, like action density or comparative value of green (how it's low value early, while trying to get your engine running but high value later, when trying to keep up for the end of game) don't belong here
Mechanics that fundamentally alter rules of the gameOfficial mechanics that do this are Landmarks and Night cards
Mechanics that change how existing things interactNew non-frivoulous types, etc
New PhasesPhases beyond the AB(N)C
Alternative Money / Alternate CostsA new, different currency to try to wrangle. Debt and Potions would both go here, as would how Peddler changes costs, Grand Market has restrictions, etc.
Not Otherwise ClassifiedA mechanic included here probably suggests a new category is needed