That generally provides enough core cards to have the deck function while adding a solid amount of variance
Goldspan Dragon isnt necessarily a combo with the magecraft creatures (though it can be, to some extent), but the key here is that they both want the same type of card in your deck
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Graywater's Fixer lets you extract a lot of extra value out of your dead creatures
You can put Doubling Season in this spot too, but Aether Refinery is a lot cheaper moneywise, and does what youre wanting, while providing an energy outlet for big tokens
Dogged Detective returns from the graveyard when your opponent draws their second card, making it a highly expendable piece of sacrifice or discard fodder