I think you have something with the whole coliseum idea, but I don't think we are going to be fighting waves and waves of enemies as the entire raid. (Or at least I hope we don't) but maybe we will for a part of it. (Kind of similar to when we had to fend off waves of the vex while defending the confluxes and we had to go through waves of the oracles) But I hope that we have to fight our way to that coliseum and then we have a few waves of fallen before we have the final boss wave! And after that we have to escape on our sparrows to get to our ships while we are being chased by fallen!