While you can overall take different upgrade paths in Boss Rush, some characters have just a little more than 8 gray upgrades available, leading to the final builds being a bit too samey than they could be and hampering the mode’s replay value. Which is why I think there should be more gray upgrades available.
These gray upgrades may even be pretty generic all-char ones, like making counter-hits do +1 damage, as long as they increase variety of possible upgrade paths.