don't listen to the negative criticism, this is a very creative idea for Halo, I thought about this for quite a while myself. It should have many many elements from Battlefield, such as:
-many variants of weapons (from each and every Halo game as well as new weapons/variants) as well as the vehicles and its variants (ex: different warthogs)
-different classes for different factions, with their own set of weapons and characters (UNSC - Marines, ODSTs, SPARTAN-IVs, so on. Covenant - Sangheili, Jiralhanae, Sangheili Rangers, so on.)
-different playable factions, such as the UNSC, the Covenant, the Insurrectionists, and the Heretics.
-depending on the map, it could include which faction fights which, making a large variety of battles. These include:
UNSC vs UNSC (simulation)
UNSC vs Covenant
UNSC vs Insurrectionists
UNSC vs Covenant Storm
Covenant Loyalists vs Covenant Separatists (could have a map on High Charity during the Great Schism as well as other maps)
Covenant vs Heretics (could have a close quarter map on the Gas Mine in Threshold from Halo 2, and other maps of course)
DLC - Forerunners vs Ancient Humanity (One out of possibly 4 classes for the Forerunner can be a Promethean Knight, aside from actual Forerunner Warrior-Servants with Combat Suits)
If the Forerunners don't become a playable faction sadly, then the Forerunner weapons can be neutral weapons that can be selected in all classes of all factions, just like what Battlefield does.
Hell, even on certain maps, there can be Flood outbreaks where they begin attacking players in their path, maybe starting off with Flood Infection Forms popping out of Egg Forms, or even Flood Pods shooting down Flood Combat Forms from a flood-infested frigate that just arrived above the map through slipspace.
And finally, there can be many different gametypes but one main playlist that is similar to the game Section 8, where it combines a bunch of objectives into one (kill a VIP, transport a vehicle to a certain area, defend a base, extract classified intel, prevent a MAC blast shot from shooting down one of your team's frigates by hacking a cpu system, etc...while all in one match) while still killing enemies for points, which all accumulate points until a team wins.
I believe a Battlefield style Halo could VERY creative and could be one of the most unique video game experiences anyone has ever witnessed, Halo fan or not. (hopefully very probable in the future, maybe that MMO experience could come in handy for let's say 164v164 matches)