An ability that triggers when a creature becomes monstrous won't trigger if that creature isn't on the battlefield when its monstrosity ability resolves
If the card has any mandatory additional costs, such as that of Spark Harvest, you must pay those to cast the card
For example, if you sacrifice a nontoken Clue artifact, such as Candy Trail from the Wilds of Eldraine set, Piper Wright's last ability will trigger
What competitive formats is Thrill of Possibility played in
Hydro-Man spent over a year with "become an Island" text until, near the end of the process, our kindly editor reminded us that would cause him to lose all other abilities, which causes numerous weird rules interactions
With just one Thassas Oracle or Leyline of Anticipation in play, Callaphe is a 4/3 that protects herself and your other creatures for three mana