I'm actually kind of excited now about the shards. Jeskai is a blue, red,
white shard with a spells matter mechanic: I am sure to (finally) get some awesome white build arounds for that theme.
Prowess itself seems nuts considering how often it triggers; it has the potential to be a fixed version of storm, and could solidify spells matter as a real cube archetype.
Consider this slightly expensive guy:
When you combine him with other tempo cards:
You get a threat that can sometimes just combo out and end the game in one attack, conveniently at the 6 mana point.
And he even scales with power level:
I'm excited to see where they go with this.