The reality is that fast mana cards are so good that they are banned or restricted almost everywhere else
Herald of Eternal Dawn can be a nice surprise threat, especially against red aggro and creature aggro in general, as these decks regularly lack the means to get rid of a 6/6
To get the most out of Goldspan Dragon, you can consider using Gadrak, the Crown-Scourge, Magda, Brazen Outlaw, and Brass's Bounty.
// Esper Terra Legendary Enchantment Creature Saga Wizard 6/6 (As this Saga enters and after your draw step, add a lore counter.) I, II, III Create a token that's a copy of target nonlegendary enchantment you control
It can come out larger the further you are into a game, and benefits from other cards that can put counters on it