Remember: a new big turboprop is not competing with A320/B737 (which are not replaced till 2020). It is competing against C-series geared turbofans.
As for speed - why 676 km/h? I think A400 is faster (slightly). Tu-114 is fast, too.
If you start the design of, say, ATR102 from two big turboprops of Ai-400 (with counterrotating props) - what would be your suitable MTOW?
As for skylights, remember that Tu-134 has the skylight in a toilet...