Give uncollected rares in Standard/Historic event
Since the "Standard Event" and "Historic Event" cost gold, and the games are not easy (most people play tier1 decks there, or close to it), it would be nice if the rare rewards (for reaching 5+ wins) would be uncollected rare.
It would make those events a bit more appealing, because right now it feels underwhelming to perform well in such an event, and not see any change in your collection (for people who already have a majority of the rares in standard).
Compared to e.g. a draft where you'll likely increase your collection by a lot if you get 7 wins (in addition to making a profit on gem, which you do in both events of course).
Or maybe an alternative could be to have the first rare be from the latest extension, to increase the chances of it not being collected yet (but this solution works only for the standard event, not the historic one).