SM64 graphical style and music would be fantastic, if not for nostalgia, at least to bridge the gap between the classic 2D styles and the polished 2.5D of NSMB.
Plus I think it'd be interesting to have the trio of SM64 caps included for "unique to this style" play. I think it'd be really fun to revisit, even if only in 2D sidescroller mode, the Metal Cap, Vanish Cap, and Wing Cap.
More likely, SMB2 would be used, and while that WOULD be fun for sure, I think SM64 has ultimately more unique additions to include and more to bring to the table. But that's just me.