Either.
Like because the games will largely be 2D anyway, and really if we wanted to play old games, we'd use an emulator.
The idea behind the competition is to remake an old 2D game, in 2D, or in 3D - the important factor is to improve on the original somehow. Making a 3D version of a 2D game tends to be the way it goes with these competitions.
Remember though, that 3D graphics do not automatically mean 3D gameplay, it's just that 3D is a good way to expand on visuals, the game logics could all be regarded as 2D most of the time.
For example, I made a Jetpac remake a while ago, and used 3D models and levels and stuff, but it was still good old 2D Jetpac at heart.
Aegrescit medendo