Article explaining how adventures came to be.
* First came the roar mechanic in Ixalan, which acted like adventures do now more or less, but only went on expensive dinosaurs. You'd hear the dinosaur roaring in the distance, which would do something, and you could cast it later from exile.
* Later, they put channel from Kamigawa block into the set, but that didn't add enough decision points, so they added the ability to cast it after you channel it. Then they wanted it to be counterable.
* There were both adventures, and a splashier mechanic calls quests, which was a discarded Zendikar mechanic. People confused the names of the two a lot internally, especially once they tried switching them around.
* As part of Project Booster Fun, they wanted a mechanic that could get a fancy frame treatment. This led to adventures getting the "creature with a spell" aspect, which also addressed them wanting adventures to be counterable. This also led to quest's removal, as it didn't need to be the splashy mechanic anymore.
Merchant of the Vale looks like it could be good in nonrotating formats. In Limited, it not having to tap for its ability should be big.