It´s far easier to add more enemies to the pod in random generated maps like in Xcom tho.
Sven stated in the interview that the game engine and UI will support 6 characters and we would have mooding tools whenever, but the thing is just adding more characters to the party is not enough. You have to be sure that the dialogues, flags and banters will fire accordingly, the characters do not get stuck in cinematics and possibly tweak the enemy encounters so you do not overwhelm any enemy you find.
Right now we know nothings so.. lots of ifs he he.
At least if they´re going to add more enemies and more difficult encounters in the harder modes maybe you can mod the game to have 6 characters and play in veteran or nightmare and the game would be challenging. Still the question if the extra characters would add something to dialogues and cinematics too.