Is it safe to spend zenny or should we save it for something? I'm starving for more cards.
If you're roughly 50% to your next legendary (i.e. 15/30 packs) then I'd personally buy CORE packs individually until you open your next Legendary and then stop buying them to save for the next expansion.
I was at 23 packs and ended up needing to buy 7 packs with Zenny (absolute terrible luck). But I still have 6K Zenny saved, so I should have plenty for the next expansion!
Also, I would expect that the next Season Pass includes missions for rewards of Zenny and expansion packs.