Inside the church on Mercy. Make the side entrances a tad bigger, that’s where players spawn. Close the Gate, and remove grenade spawns.
One side stairs of Blood Drive, both mid and top level. Stairs leading to bottom will be blocked off, spawn area is blocked off, and half of the grenade area is blocked off. One team spawns up top near the normal spawn, one spawns low near grenades. It rotates after each round.
Bottom section of Foundation, where frags and the 4 pillars are. Block off openings to Boom and Tbow, add a short 1x1 block in front of Tbow steps, remove grenades, and have teams spawn on mid level with a wall behind them blocking spawn areas of the map.
From Library, a Judgment map, this is just the block of the the book stacks to the one side of the map, where the ammo box would spawn. One team spawns on either side with their back to the enemy and the stacks behind them.
I like those and the OP’s ideas (except for Reclaimed Barn, not feeling that one). Would make for some great rotating maps for 2v2 and freshen things up as I couldn’t imagine playing only Boxes for more than 30 minutes at a time.