The only issue would be how to handle loot and xp.I would play with no promise of either but some will complain if it gives too much or too little. finding a balanced way to reward players for putting so much time in will be a challenge. Im thinking higher tier drops after every exponentially difficult wave completed.

This would be amazing and doing loot and experience would be simple. EVERYTHING based on progression. For every cleared wave after say wave 10 yields loot greens at the start and then every 10 levels after that ups the rewards in the case of loot rewards. For experience do the kill experience as the reward for that so innately the farther you make it the more you get. Have no cap on number of waves and scale HP of mobs as you get higher add shielded enemy's as well. But I would steer away from damage scaling because damage is already high if your not careful. You could also add one of the heroic modifiers every 10 levels like arc burn and so on. You could really do alot with thus idea.

I agree 1000 times