add achievements for all paid towers and heroes (but not leveling them up to 10th). The achievements could show up only if the appr. item is bought, not at all times. And if you add "Reset Achievements" button you will be the best. I don't want to play again the basic levels but I love the tasks
You can add 3 levels for all special abilities of towers as tasks - i.e. Orc Shaman - 1) Heal 10 000 damage, 2) Kill 100 enemies with meteors 3) Shock 100 enemies with sparks etc.