That would be simultaneously rediculous and pretty fun. But the amout of griefing would be monumental.
The map would be like Forge world, friggin massive and impossible to traverse without a vehicle to get there, and die from a tank.
The weapons would be picked over like a carcass in the desert, you'd be lucky to get there to get the ammo lying around.
20v20, even if possible, would either make areas turn into massive battlegrounds, where two full teams would have slugging fests until one or two on one side survived the massive onslaught, and then would quickly die from the enemey's counter attack, making for a massive stalemate or one team would bulldoze through the other, and camp the crap out of the other team.
The perfect map is impossible to simply put it, tanks and turrets do not mix, nor do close quarters and vehicles, the map would seem schizophrenic, unable to deside on the play style, and players would find themselves with close quarters weapons in open vehicle terrain, or with rockets in tight halls, stuck on the part of the map that suits their weapons until they die, or find some to possibly survive somewhere else.
The assault part at the end would be a bloody stalemate or more steamrolling again, the bomb carrier would be killed so much, it would take an army of bomb carriers to get it across a map of the nessisary size.
A weapons shop is dumb, this is halo, not Killingfloor(look it up), points would cause better players and teams to have a massive advantage over the other players/team. Buying weapons means arses could grief, camp the store and teamkill for fun/weapons. Droping vehicles would result in having it stolen by friend or foe, or camped by enemy anti-vehicle weapons.
Communication and any form of strategy would be near impossible, like you said.
The idea would be fun, but horribly flawed. I can say that Halo is not built for this type of game, and idolizes the individual player, compared to teamwork. The whole thing would either drag on, or would be onesided. The superior team would be very appearant, and close games would be a never ending stalemate.
Its good, fun probably but not right for halo.