Cart synergies, e.g., anything bordering a sharpshooter cart gets increased range, witch carts grant small AOE buff, archers grant attack speed, crossbow buffs damage, etc.
Re-ordering carts for 5 gold per move
Teleport pads where the train has to be stopped but can move you to the other side of the map after 2-3 seconds, maybe has a long cooldown to make it very strategic
Upgrade/repair base defenses in shop (damage, hp, rate of fire, range, etc.)
