Hehe I think this would be pretty awesome. You'd have to vary the maps/enemies though. Thralls that explode, thralls that poison you when they attack, heavily armoured thralls, jumping thralls, thralls that throw debris at you... I'd play it! You could even have different modes such as your standard survival mode, defend the helpless exo hunter who didn't have time to explain, maybe even a tower defense kind of play style. I honestly can't see it happening anytime soon but I hope we get more interesting scenarios. I've yet to even come across a 'Defend the ...' mission that actually involves protecting a vulnerable object.