The flash phase could exist at the beginning of the opponent's turn and that would make a lot more cards useful but then once your opponent casts a creature especially with haste or buffs a creature or has a creature's effect activate, the opponent may want to cast something as well. Perhaps after a support hits the board but before it can do something powerful like Song of Creation can. Here's the problem though- How many times during your opponent's turn, do you want to see the option pop up for you to possibly use one of your several flash cards? 3 times per turn? 4? For each card with flash? The only other alternative I see having an okay outcome would be for the player to be able to click on the icon of the card in hand when the hand is closed, to unlock it as a means to say "I'd like to use this card now" and players would have to be on there toes ready to click at the right moment. Then the option for you to pause what the opponent is doing would come up so you could play the card you tapped (which you might not get off in time). This is a lot of interruption going on though, like the stuff that likes to freeze the game like clicking the hamburger button when other actions haven't resolved. If that can be fixed than perhaps this is the best option for flash and the costly time and unnecessary clicking of flash cards when you aren't interested in using them can be removed in exchange for the tap of a card icon. Even then, matches will begin to take a lot longer and the AI will likely put up no challenge way more often because players could put a stop to anything happening. I kind of think flash (even if done right) will crash this game for it will make completing the many events that take forever already, take even longer. If the AI gets to use flash as well, it will be a lifetime before anyone gets anywhere.