I've spent most of the year working on my create a quest project which is an action/rpg maker. Lately I've thinking about implementing a mini game maker that would by either a card game or some sort of puzzle game. Do you think it's appropriate to include a mini game maker? What are some mini games that would be good to include with a main editor?

What do you mean by "mini game maker"? You mention a card game etc but how will the user define the rules of the game? How much flexibility would they have to actually design a mini game from scratch?

By mini game maker I refer to things like the card game in FF8/9 or the hacking in Bioshock.

The only way I can think of doing it at the moment would have 1 or 2 set games that the user can just customise.

In my opinion, attempting to define a type of mini-game that will be present in the end-user's game should be the responsibility of the end-user (the game developer using your tool). If your Action RPG Maker has a sort of scripting system that would allow the user to create non-standard behavior such as a card mini-game, I would think that is the best way to go.