It would be a fun experience, but I would get my laughs by running across the battlefield every now and then to draw the fire of different enemy groups into each other. I could get the flood to start a charge, and then run into a pack of brutes. Then I'd sit back, pop some popcorn in my Mark VI helmet, and snipe whoever got too close, or whoever I just didn't like at the time.
If you had flood, covenant and sentinels altogether in the one fight, you'd be able to sit back and let them kill each other an then mop up the rest. A section with heaps of covenant enemies onto you and a bunch of marines would be fun though. If you had heaps of grunts and the grunt birthday party skull on it woul be awesome.
In H2 modding days, there was this big AI campaign mod that did this. Laggy as hell, but fun. Especially on Legendary. In Halo 3, that would have been fun, but the theme for the game was that they were the bigger, stronger force, and the small strike team managed to beat them by attacking the core. A massive epic battle would have disrupted that a bit.
I wanted to use the Splazer more than twice throughout the campaign. A few more of the sweet Brute encounters like the end half of Tsavo highway would be sweet too.
I wasnt expecting forge but when i read about i loved the idea but the only downside at first was the items we could spawn. Why? you couldnt create a map on guardian out of rucksacks so why have them? i wanted it so we could be able to have walls and doors that fit to the original maps like the doors on Blackout(not the same ones obviously ). that would be epic because imagine the variety we would have on FH and Bungie.net instead of constant maps on old maps like Sandtrap , Last Resort and The Pit and the New ones like Foundry and Avalanche.