They ARE making a diablo 3, except it has a different name, but it is suposidly going to stay, at least semi-similar.
In press released comments from blizzard, they plan to "sometime in the future" return the the PC world and make starcraft 2 but that does nto mean its being worked on, or will be for years, and they can change there minds.
I think they waited to long and now its going to be hard to make a game that keeps up with the SC standerd. If thye made it after they could have upgraded graphics, made new units and a new storyline and used the same engine and everything. To long has passed and if they make it now people will expect a full redo of the game from units to game engine.
I STILL play starcraft one and expansion, and would like for them to make Starcraft 2, but take an extra full eyar then they planend to perfect every bit of it, to make sure it lives up to its name. I don't mind waiting for something that great.