Could do "basic" upgrades to affect stats
+5% accuracy 0/5
+5% hp 0/5
+5% attk spd 0/5
+5 armor 0/5
+0.5 bullet spd 0/5
so they'd cap out @ 25% which is a decent starting point
more expensive ideas'd be like.
+3% chance to get gold attribute cards on lvl 0/5 (caps @ 15%)
+1% chance to double card bonus for the level (caps @ 5%)
+1 banish 0/3 (so we can banish a card we dont have to see again if it doesnt fit our builds)
somethng like that. that way you dont have to make a dozen "skill trees" for each upgrade path, you just change the chances we get something better on lvl up.