So if a firefight/horde/wave mode comes out for destiny I was thinking in terms of gears of war 3 where the enemies move up towards ever increasing boss difficulty.
Historical battles in destiny would cater perfectly to this. I've seen others mention the twilight gap before. Waves obviously would cater well to this. You can mix or have separate houses attack. Major offensives from the house of kings as the leaders and bosses like mass tanks. In waves they could make them even more challenging with modifiers or easier by for instance calling in lord shax with his fireteam.
There could be a final line of defense that is constantly overrun but provides super for a line of Titans that you have keep alive as you go through. There's a lot that can be done.
What I haven't seen is the attack on the moon mentioned. This could also work out well and have multiple phases. Storm the hellmouth or defend the beachhead (Moonhead?) or stand your ground. Culminating to the final wave where instead of survival and counterattack like the ending of the twilight gap. It's survival and retreat as crota attacks. I'd imagine a final wave where you have to hold off Crota long enough to evacuate as he smashes through everything you worked so hard to take throughout the game. Really adding weight to how hopeless their situation was.
If bots are ever included then we can also have more "heros" taking the field with us.
Probably more for destiny 2 but just a few thoughts. A historical battles dlc would be amazing