I've noticed that the enemy likes to hide behind walls, and there is occasionally no cover to close in enough to throw a grenade, usually without exposing yourself to enemy fire.
Mortars as heavy weapons would neatly solve the problem, while being able to potentially punch through floors. They'd be very expensive to use, and possibly immobile within the battle space, so they don't step on the toes of grenade launchers.
Having multiple round types would make it a useful tactical weapon. Large area smoke rounds, incendiary, and naturally armor piercing. Possibly even a camera round to be dropped into an area you can't see. It doesn't have to last for more than a round.