We'll have to wait until we get our hands on one to know for sure how much this impacts the playability of the cards, but they'll for sure be a popular variant given the exclusivity and the fact that they are associated with rares and mythic rares only
Elesh Norn, Grand Cenobite is a seven-mana, 4/7 Phyrexian Praetor with vigilance that grants creatures you control +2/+2 and creatures your opponents control -2/-2
On the one hand, I wouldnt recommend playing Kaza in most competitive settings because its not exactly a powerhouse card
It has payoffs for two of the three types of modified, and then limits the counters to only going onto creatures who already have them
The Soul Stone is a black legendary artifact, and the other Infinity Stones will almost definitely come in Red, Blue, Green, and White versions
Throw in a few Mounts to take it from A to S tier