That's archetypes, not mechanics.
Mechanic I'd most want to have back: blacklisting.
Archetype: spies
I think silvers are sorely missed as well. They really helped bring the deck building process together. Now golds just don't feel like golds any more, if your going to put a purple border on a gold card why not make the border silver? Just feels stupidSilver cards. No, seriously. Yeah, lower value golds could be classified as "silver". It's highly irritating navigating the deck builder when the options are gold and bronze. I want my silvers!
Gold immunity is off the list. I imagine having such an option would be met with resistance. Still, it's probably a worthy option. Contrary to certain opinions I never felt it was problematic by itself. Furthermore, it differentiated golds from lesser cards in a unique way beyond, "It's better.". I think the only archetype truly fucked by gold immunity was SK Axemen. Otherwise known as Greatswords now.
Haunt I've liked immensely as well, especially with so much card art for wraiths, nightwraiths, noonwraiths and barghests. Hell you could have gold support with the likes of The Beast and such as well. There's a lot of potential for a future archetype there.
Thronebreaker has used artwork and ideas that will be implemented in an upcoming expansion, probably under the vampire banner (while technically not having anything to do with that). That's what I think.