"Retrofitting 3D is actually harder than building 3D into a game from the outset. One of the big issues is that all of the game assets were designed to work with only one HD output. Being able to render them twice to make a 3D output can be a challenge."Working in 3D from scratch changes things though:
"We're actually surprised at how much time game designers currently spend working around the lack of 3D in 2D games. When we first started working with 3D features, we thought it might introduce difficulties in game design, but actually the opposite is true."
"Because 3D provides benefits such as depth perception, any spatial tasks - for example building a level using a construction kit - are much more intuitive and don't require things like grid overlays."