Thanks for playing that far! Great question. Leftover gold already turns into crystals when a run ends (1 crystal per 40 gold, shown next to the crystal counter now). And in today's update (1.2.0) there's a new use: after the wave 20 boss you reach a Camp where you can spend gold on repairs, an extra card, or overcharging maxed cards. Past that is a new region, Frost Peaks, with new enemies and a boss at wave 40. Let me know how it feels!