The TVTropes Trope Finder is where you can come to ask questions like "Do we have this one?" and "What's the trope about...?" Trying to rediscover a long lost show or other medium but need a little help? Head to You Know That Show and try your luck there. Want to propose a new trope? You should be over at You Know, That Thing Where.
Surprisingly Improved Sequel - First game was considered mediocre or worse, so it's surprising that the sequel was such a big improvement.
Even Better Sequel - First game was good, sequel is better.
The Pennyfarthing Effect - "Obvious" innovations were added to the sequel, making it seem like the first part went out of its way to have clunky game mechanics.
There was a TLP suggestion for "Experimental Sequel" a long time ago.
Plain first part, more innovative sequel
Primarly in video games, some sequels don't change the core gameplay, but add new elements and mechanics to increase the depth and variety in the game, sometimes to the point that the first one looks now a bit uncreative.