It's definitely not your traditional backpack, but I bought this last year and love it:

Nomatic 20L Backpack

It's a bit pricey, but it is very well made and has a well thought out design. All the main compartments zipper down and fold open for very easy access. It has a zipper expansion that add 10L of storage (this is how I normally use it). To your issue with the straps - this backpack has a cover on the back where you can tuck the straps away so they don't drag on the ground. The bag has a nice flat, rectangular bottom so it sits upright nicely when free standing or when hooked to my suitcase standing upright when I'm not walking. Also, most backpacks, when they are hooked to your suitcase J-hook, make it nearly impossible to access your laptop/tablet compartment since the hook on the backpack is in front of the laptop compartment, so you have to take the bag off the hook if you want to quickly pull out your tablet. This bag has the hook at the very back of the bag behind all the compartments, so if I say want to sit down at a gate waiting for a flight I can just set my suitcase where I want it and grab out the ipad without having to take everything apart and put it back together before I leave.