I know it's not the "popular" answer, but if you don;t yet have 3 ea of all of your key Bronze cards; start there. The consistency will really help decks while you save up for more silver and gold cards. If you are bouncing between multiple factions, you may want to lean toward gold Neutral gold cards (as they will work in any deck) and neutral or faction specific silvers. I'll try to add some less obvious choices.
Neutral Golds to consider:
Geralt: Igni - Single Row Scorch, if used right, but can backfire (esp. in 3rd round)
Ciri - can swing card advantage the round you expect to lose.
Renew (may get nerfed soon) - reuse something, when needed (can backfire if no golds are played early)
Yennifer: The Conjuror/
Triss: Butterfly Spell - Opposites of each other, depending on if you prefer a buff or damage archtype
Regis: Higher Vampire - can remove a key bronze from opponent's deck, solid body
Silvers to consider:
Neutral:
Alzur's Double Cross - Build your deck with one (or two) clear targets, and you can have an extra way of getting a key card (such as Nennekke in NR)
Olgeird - Keeps coming back, can be good in Base STR buff strategies
Summoning Circle - very underrated (IMO) as you can gain access to out-of-faction abilities. Example: Stealing a Clan Tuirseach Skirmisher with Vivovaro (so the SK opponent can't res) in one round, then copying their Priestess to res your stolen skirmisher in the last round...
Monster:
Frightener - Move out of Weather (or other danger - or to consume another row) and get a card.
Giant Toad - Consume from hand and draw. Also great with things like Olgierd, or just to replace something that won't be useful on the field.
Nilfgaard:
Cynthia - Great first round, first turn card. Often comes down with 12-14 STR.
Fake Ciri - The Spy. If opponent doesn't answer her early, you are likely to get the round.
Northern Realms:
Prince Stennis - He'll eat removal often; but for opponent's that don't realize it and kill him quick, he can get out of hand in a machine deck.
Reinforcement - Get the tech you need. Now.
Scoia'tel:
Braenn - if you like boosting cards in hand, she can be harsh after some buffs.
Toruviel - Many opponent;s will see this coming, but the "math" can be difficult to know what yo uneed to win, especially as units move in and out of her row.
Skellige:
Champion of Champions - Underrated Skellige. Especially great against weather, as he'll absorb more damage then a late-round Mardroeme returns all that nice base STR buff he was getting (but you never saw because of the weather)
Sigrdrifa - All around good Skellige resurrection.
