Taurean Mauler is one of the best options, as its stats will constantly increase when your opponents cast spells
For my tastes, as guidelines I think are worth sticking to, a keyword mechanic needs to be doing offloading in the rules text that makes things more convenient , it needs to have human-understandable reminder text that isnt just the rules text written out, and it needs to be something that other things can usefully reference
Blitzball has already been hinted at, but I know I'm personally hoping for the likes of Triple Triad, Tetra Master, and the Coliseum
Flavor Text Wenn Gebete zu Versprechungen werden, kann ihnen nicht einmal das Schicksal widerstehen
Sozin used it in his campaign of conquest, leading to the destruction of the Air Nomads, and a century later Fire Lord Ozai attempted to do the same
Since these decks almost always have access to white, Cloud certainly shouldnt struggle to find a home