I think that some people definitely don't understand, or just shrugged off the actual question, which clearly implies that the titles you suggest would mean that the original was mediocre. Yet, I see games mentioned such as Half-Life? Borderlands? Mortal Kombat?
Even if the sequel was, for example, a "quantum leap" over the original, was "better in every way", doesn't mean that the original was simply mediocre. Seriously guys, the original Mortal Kombat, alone... do I even have to start explaining how by itself it was already a quantum leap on its own when it was released and what it provoked within the video gaming industry? Does ESRB ring a bell? And do I really have to explain as to why the original Half-Life was the absolute FPS revolution and evolution since DOOM and Quake?
If you really do consider that such original games were already mediocre, or mediocre "IP's" during their own times, then either you just weren't there when it actually occurred, or again, you just don't consider the actual OP's question when you post your suggestions. The question is not simply "name sequels that were better than the original". If I was you, OP, I would probably just change the title itself, since obviously lots of people around will just come in and pop some random sequels without considering the actual question.
There let me have my take on it...
Homeworld 2, yup, 'cause the original was totally mediocre.