OOH! The variability in the upgrade menu is INTERESTING. Not sure how I feel about only _ever_ being able to add one extra slot, but let's see how it goes.
Viewing post in Upgrade rework, Better tooltips & More | Patch Notes comments
I definitely find my self holding onto the explorer for longer, making it free with the upgrade and buffing it with other cards.
Temple also becomes a good pick: I often steer well clear of structures because taking up a slot with no attack action is such a penalty.
Hard to imagine myself wanting to take piranha, unless I'm going for an "on death" strategy.
Sun stickers become more of an appealing pick for sure.