If you're going to go that far, you might as well make it truly unique and simpler at the same time:
"Gain a card that is not a victory card."
It's more powerful in terms of card gain, but puts the card in the discard (unless you have liopoil in hand). But gaining anything up to a cost of $6P and topdecking it is pretty powerful.
If you want it to be able to gain Potion-cost cards, then this idea (gain anything that is not a victory card) looks good. But then the card loses the ability to topdeck a gold, its original function. And Potion cost or >6 cost cards may completely outclass Gold leading the card even further from the original function (e.g., Possession, Platinum, or King's Court).
Donald had talked about giving it +3 Actions (instead of +1 Action), and removing the +2 Actions on Trusty Steed (replacing it with something else). I think you could do that and leave Trusty Steed as-is, though.
On some boards it would be nice if there was more than one +Actions Prize but +3 Actions and topdeck a gold don't go together thematically, and I think that version would be gained more for the actions than for the Gold. It would certainly make the card stronger and more interesting, though.