Early NES sequels thought a new game had to be completely different. Playing it safe seemed like a ripoff. The first 3 of these NES games changed for the 2nd title then went BACK to the formula of the first game.
Mario 2 US - insane 2D platformer. Completely different mechanics. Lifebar, picking up objects, multiple characters. Even though in Japan they got a "safe" SMB2, in America, marketing gave us this out of left field game.
Castlevania 2 - 1st was a 2d action game. 2 is a Metroid style game with a day and night cycle and very convoluted puzzles and item management.
Zelda 2 - turned onto a 2d action RPG with XP points. XP were a relatively new idea in NES games.
Teenage Mutant Ninja Turtles 2 - first was a weird 2d action platformer with an overworld. Difficult in its own right. The sequel is a classic beat'em up.
Wind Waker - everyone was expecting a more "mature" Zelda game since it was the xtreme 2000s and Nintendo released a video demo of an adult Link. Instead we got a cartoon adventure that wasn't fully appreciated till later.
Metroid Prime - abandoned the 2d style Metroid genre it created, for a First Person Scanner. Shooting is involved but the scanning was an amazing part that became the focus after Miyamoto suggested it.
Star Fox Adventures - on rails space shooter became a 3D action adventure. Wasn't well received but the Star Fox franchise is stagnant and they tried something new.
Final Fantasy 9 - you could argue this was safe or wasn't. But a choice was made. Breaking from the futuristic and very successful high-tech style of 7 and 8, FF9 was a purposeful throwback to the medieval fantasy style RPG. Bringing back the black mages from the older games, having a regular knight as a character and then making the main characters childlike and Disney safe princess and thief. This was a goodbye letter to the old style as the future was going to be more mature and pushing the boundaries.