This list is based on the competitive side of things, looking at the cards that have had the biggest cross-format impact, but especially in Standard, hot on the heels of the World Championships
I dont know of many cards that can punish greedy deckbuilding this cleanly
The One Punch Man set, which launched in English and Japanese at the same time, got a ton of buzz without breaking the games balance
5 Forcing tribal spells because they are tribal, or having tribal effects just because they are tribal is bad design unless you plan to lower the whole powerlevel obscenely
Solo battles, random multiplayer, and private matches are available
It doesnt take much before Cori-Steel Cutter runs away with the game