For track 1, I lean toward the 10x10. Hadn't considered the 9x10 size; it could give some interesting games. Again, here I'd be inclined to put the pawns on the 3rd rank, and make them only 1-steppers. But then the pawns could not meet in the middle on turn 1. This alone may disqualify any such game from serious consideration. So we're left with 8x10 and 10x10, both of which can easily and naturally give 4 squares between pawn rows, maintaining a key aspect of western pawn formations, as well as the little wrinkles of the double first step and en passant. This should keep the orthodox players happy. 

For a serious first pass at 'the next chess', I strongly suggest grabbing an old idea, and dropping the standard 8x8 setup in the middle of a 10x10 board. By the way, understand I don't believe there is such a thing as the next chess, but the concept of a game sufficiently like chess to grab the attention, and playing time, of FIDE players is a truly excellent design challenge. Back to the 8x8 setup on the 10x10 board. Here we have the basic design of chess and Christian Freeling's Grand Chess. Now the important question becomes: what minimalist changes can we do to this setup to create a game that appeals to a wide range of chessplayers? 

Break the question into steps. Do we add 2 pawns to cover the front? What pieces might we slide back to the last row? Then maybe we might look at new pieces, as it doesn't feel right to increase the number of any particular piece, unlike with pawns, where adding 2 is not only natural but obvious. What sorts of piece[s]? Since there are no more simple infinite sliders, it's going to be either a complex infinite slider, or shortrange pieces. Here, my personal preference says shortrange.