twenty forests) is a tried and true practice opponent, and having a copy of that deck is beneficial, but were talking about a real deck here
There will be more cards of a rare and mythic rare power level, but adapting to that (making sure players have more answers at lower rarities) is part of how R&D is adjusting our set designs
Reflecting Mirror While having access to a tool that lets you change targets at will, Reflecting Mirrors downside is that it only works whenever a spell targets you, meaning that it isnt very good at protecting your board, even though it's a decent artifact against hand hate
Instead we're talking about Christmas cards
For example, Delver of Secrets has a triggered ability occurring when you reveal an instant or sorcery from the top of your library while Bloodline Keeper has an activated ability with a cost to transform it (in this case, paying a black mana)