Interesting, this one did pop up on my radar a while back, i always keep my pulse on the playtester space :) Although cardtavern one strays away more from the free form play that simulates a kitchen table game of magic, piles and structured zones ect, much like untap.in has (as i mentioned, users want fast structured play and QOL mechanics built in). The ops vision seems to be its a table with some hide/reveal and shuffle mechanics, do what you want after that. Which i appreciate, allows a bit of fun.