The most notable card was "Horus the Black Flame Dragon LV6", which could be used as a generic Tribute Summon which could eventually Summon "Horus the Black Flame Dragon LV8", which could negate the activation of (almost) any Spell Card whenever the player desires
A lot of balance and care goes into choosing the cost of a card, more than I can express here, but looking for similar effects on existing Magic cards can give you an idea of a fair costfor example, a ramp spell that puts two lands into play ought to cost 4 mana, per Explosive Vegetation
7 Snorlax - Jungle Set Snorlax is a fan favorite, but while most players held onto the original base set cards for the large sleep Pokmon, there is value in every version of his card
The nostalgia pool shows that there are many different ways to remember the game
Fake cards Light will often shine through, sometimes faintly revealing the cards image from the other side, because counterfeits skip this internal black layer to cut costs
That being said, the existence of these products might be a necessary evil for MTG Arena