Take any level and put key holes in about 2/3 of the normal spaces. So 1/3 of the map is red, 1/3 blue, and the rest is just normal map.
The game is played in 2 parts. First you spend all your credits taking turns placing units on the board. Each unit will sit on 1 key location.
Once all the credits are spent you then play the game as normal but with a whole army of units.
It may need some first move balance, e.g. movement of the first player is halved. E.g. 1 move unit can only move once (could still attack), 2 move units have move range 1, 3 range move 2, etc
It would be a more chess like game. You can see all your opponents pieces and would need to out manoeuvre them, sacrifice small units to tempt bigger units into range, etc