With the suggestions, there are some problems:
- an engine to add new cards of your own creation
That's extremly easy to DoS, or it's extremly hard to prevent to DoS, because you could just include cards like:
Do this 10^214 times:
put your draw deck in your discard
shuffle your deck
and it's only limited by creativity.
As the state of the game has to be calculated on the server (to prevent cheating), you would to have be very carefull that no card will take to many resources, which is an impossible task, unless you have very strict limits on what the cards could do. Which would be boring.
- an "undo" feature during play
That would of course be nice, but you can not allow to undo any action that reveals information, so that is basically no Attack (because of possible reactions), no +cards, no inspect your draw like Scout/Apothecary/etc, no buying Provinces (because of Fool's Gold). There will be probably only a tiny fraction of cards left where this would be possible, which is probably more confusing than helpful ("why am I allowed to undo X but not Y), so that it is clearer that it can be just done on basic treasures.