I think they will have to stay within the boundaries of what the engine can do. From the press release I understand tactical combat is what it is now. Less fighting, but the fights require some tactics (like the Torture Master).
Action could then be; More fighting, but the monsters are a bit less powerfull than in tactical mode.
Hardcore could bring you a lot of monsters to fight.
In addition the level up speed could be adjusted depending on the mode you are playing with. Also the effect of skills could be different. Skills that inflict damage, do more damage in hardcore then they do in tactical. Maybe they could even have a slightly different skill tree depending on the mode. I'm not sure if that is possible though.
