Well saying "hoarde mode" is a very generic saying to describe the gameplay, it was thrown around long before Gears. Gears just got lazy and didn't give it a creative name.
And yes, it would be fun for multiplayer co-op. Perhaps add an option to make it competitive such that most kills wins (though only an option, sometimes you just want to play co-op without some underlying form of competition). Also one for singleplayer would be great too, just you as Kratos. I have the single player demo and simply reset after the first wave of enemies because the combat is a lot of fun, would be nice to just have that go on and on and on, neverending.