Donkey Kong 64 took the Donkey Kong Country series and expanded it in all dimensions, including from 2D to 3D. The 3D platformer is notorious for being absolutely huge in many ways, including a plethora of collectibles and places to explore. Turns out that wasn't always going to be the case, though. In an interview with Nintendo Life, former RARE dev Mark Stevenson talks about how Donkey Kong 64 was originally going to be a 2.5D affair.
"It was a monumental task, a massive game, a massive amount of work. Also it was in development for around 3 years, the team that created DKC3 moved onto it after shipping that game, but about 18 months into development it was rebooted, the team was changed up with the leads on design and software getting replaced and the game changed from being a more 2.5D platform to what it turned out to be more in line with the Mario and Banjo structure of open 3D level that got a lot of reuse. The original plans of trying to recreate the DKC format of tonnes of A-B levels just wasn’t going to be feasible from a production point of view."