The idea of having first person 3D Super Mario games is something that sounds awesome in theory.  However, there are some would argue that in practice a 3D Super Mario game would not be as effective as having a third person 3D Mario game.  Back in the 8 and 16 bit era of gaming the Super Mario Bros series was mainly targeted for kids since they generally considered to be a family friendly games.  Back in the day the bad guys such as the koopa troopers and the gumba looked scary to kids especially when they lost a life to one of them as either Mario or Luigi.  The idea of playing a game such as Super Mario 64 or Super Mario Galaxy and losing a life by falling off platorms or diving into lava is seems horrifying.

Generally when it comes down to 3D platform titles they are generally better off as third person games because it seems much easier for players to navigate throughout various levels of each stage in a game.  Also, having games such as Super Mario 64 and Super Mario Galaxy in first person would probably terrify children despite how family friendly the series is.