Every wave could go up in drop rarity at 10 no more white engram's at 15 no more green at 25 no more blue but the legendary drops are much much rarer. And finally at 30 u have a chance to get exotics. Hell there could even be a chest after every 5 lvls that gives u a roll on drops and engram's. It can theoretically keep going till 35 where u can barely do damage to ads.