Games that didn't NEED a sequel, but I'm glad got one.
Portal: I will say that while I don't like Vavle's method of CHANGING THE ENDING JUST SO THEY CAN JUSTIFY MAKING A SEQUEL (If it were ANY other company in the industry, they'd flak for it, especially here on the Escapist. But since it's Valve, that makes it ok... -_-), but Portal 2 was a good game. In fact, it was an awesome game. I won't say Portal 2 shouldn't have happened, but changing the ending? I'm just saying, this is a bit of a slippery slope, and history has proven the game industry isn't good with slippery slopes...
The Mario games (And Nintendo franchises in general): I do agree, they weren't NEEDED. But unlike the rest of the Escapist, I'm not going to go "HUR DUR SEQUELS ARE BAD AND SINCE MARIO GAMES HAVE LOTSO SEQUELZ, MARIO MUST BE BAD HUR DUR!" The games still have quality despite all these years, there's no story to screw up, and if they're good, if they're fun, that's all the justification they need. And they are fun.
Megaman: I'm not sure which one is the one that didn't need a sequel (6 in terms of "story", but maybe sooner), but the levels were always fun, the music was always awesome, and the core gameplay was always good. Once again, it was fun, that was all the justification it needed.
Games that shouldn't have gotten a sequel. Period.
Sonic 06:I don't care if Colors is a step in the right direction. If it were any other franchise, they would not have survived that abomination. The only reason Sonic got another chance so soon was because whenever a bad Sonic game was made, the fan's first reaction was always "Thank you sir, may I have another?"
The Force Unleashed: The first one was good. Not a masterpiece, not the best Star Wars game, but it was great fun, and the little story it had at least kinda justified it's existence. This is one of the big examples on how a story can screw up the chances for a sequel: If not done right, it makes it clearly obvious that it's a cash grab. Then again, EVERYTHING about this game made it clear this was a cash grab.
I can think of more, but I think these are good enough.