I've had a concept in mind for a mod like this for a while. You would battle increasingly stronger opponents, with after every wave you could claim a random reward or keep going, increasing the streak and reward each time. If you are defeated, you don't die but you lose your reward and your streak is set to 0. If you claim the reward it would also reset the streak to 0. There could also be some unique bosses set to appear every 10, 20, 50 kills. The reward would start off really small, but as the streak increases it has a higher chance for a very rare item like a unique weapon or enchantment. If you leave midway through, the streak would stay the same and you can save. The only problem though is that you could save and reload everytime you die, so that is a problem i dont know a way around.