If BTB 2.0 is literally a larger scale BTB consisting of the same game modes we've had for almost two decades I can't say I would be thrilled. However, I doubt that will be the case and hopefully 343 can create its own little spin off while not depriving all its resources to that one single game mode as its previous incarnation, Warzone. We only have so much to speculate.
If they can get it to the scale of Battlefield but with Halo skin, then I would be very, very, very, happy with 343.Then why not go play Battlefield? go play BF 2142 or whatever that future one was....Halo isn't meant to be other games
After previous installments of 343, I understand why you would see some trends having no business in halo. Hopefully I can clear some misconceptions, the main contention was never that Halo shouldn't change or try new things but change in a way that maintains its identity. Each of the individual trend that Halo 5/Halo 4 copied altered the mechanics/formula of the game from AM, custom load outs, kill streaks etc.
A new game mode isn't as detrimental as base mechanics because it doesn't effect the whole entirety of the game. Resources and time are surely an issue but with the extra year, the rumored high budget, and multiple studios working on HI it may not be as drastic as Halo 5's bare bone launch.
Instead a new game mode offers a different way to play halo and a prime example is firefight. Firefight based on the trend of wave survival from games like Gears of war and COD ended up becoming a fan favorite. Now you argued that the map design for a large scale battle would alter the golden triangle, but that's more in fault with the map design rather than the game mode itself; Barriers, trenches, tunnels, and cover could be implemented to counter being picked off from insanely long ranges.
Now to clarify I don't agree that we would need a massive large scale battle as battlefield and just because it doesn't alter the triangle doesn't necessarily mean it would be good for Halo. But the mindset of being afraid of any change won't help Halo.