Same here, except that I also have wired RB1 drums (with Goodwood Mods drum heads) so I'd like for a way to use those as well.I would love to get it digitally on Xbox One but only if I can get the legacy adapter day one. It being available only the disc bundle makes that moot.
My Mad Catz Replica Strat (the full size one) and Logitech Premium GH guitar (assuming is works with the legacy adapter) would be fine to use as they are presumably both much better than the wireless RB4 guiatar.
The majority of my previous RB time was spend on guitar, so I could probably get by with no drums for a while, but now my kids are old enough to play with me and they will almost assuredly want to play drums.