Joined: Aug 2010
Posts: 12,760
Likes: 2,118
From: Madison, WI
Bikes: 1961 Ideor, 1966 Perfekt 3 Speed AB Hub, 1994 Bridgestone MB-6, 2006 Airnimal Joey, 2009 Thorn Sherpa, 2013 Thorn Nomad MkII, 2015 VO Pass Hunter, 2017 Lynskey Backroad, 2017 Raleigh Gran Prix, 1980s Bianchi Mixte on a trainer. Others are now gone.
I am not sure of your goal here. You specified bikepacking so is it to get a nearly horizontal top tube so that you can fit a larger frame bag? Or is your goal simply aesthetics of a nearly horizontal top tube?
A 100mm suspension frame is likely a mountain bike frame or a lower end commuter hybrid frame, and of those two categories, the fatter tire size you want won't be on a commuter. So, I think you need to search for a mountain bike frame.
And if you want drop bars on it, that shortens up the top tube a bit, so you would likely be looking for a taller steerer tube and seatpost on a smaller frame size to get the fit you want.
Good luck.