I think the ideal SUL backpack depends on your underlying motives for going SUL. There seems to be three main reasons why people take an interest in SUL:
1) Personal Challenge - Pushing limits, trying new things, challenging themselves
2) Minimalist - Wanting to keep things super simple, gear gets out of the way.
3) Max Efficiency - Lighter is faster/easier, so SUL = more efficient than UL.
For me (and probably most of us) it's a mix. The 'personal challenge' motive is a fun one, but it's also an expensive one if you need to be constantly shuffling your kit to keep the gear challenge present. I fall mostly into #3. I value #2, but not to the point that it interferes #3. Simple is generally good, but too simple can be inefficient.
I think external pack pockets are a good example that illustrates the differences in SUL style. A minimalist is going to want a simple pack with no external pockets etc (ie. GoLite Ion, Zpacks Zero w/o options). I personally think I can hike more efficiency if I have side water bottle pockets, because the time spent fishing my water bottle out of the main pack bag would be more detrimental to my overall efficiency than the 0.5oz gained for a water bottle pocket. So it's a matter of style and personal preference.
With that said, for me the ideal SUL pack is:
Fabric: Either 1.5oz cuben (lightest) or a nylon/cuben hybrid (bomber)
Hipbelt: No. It's normally too constricting and unnecessary.
Sternum Strap: Yes, I like it for keeping my pack stable when I'm ducking under trees or scrambling a ridge.
Rear Pocket: Yes. I like a mesh one for either drying stuff (sunny day) or storing my shelter (wet day). With a rear pocket for my shelter I can fully pack up and close my pack inside my shelter before heading out into the elements.
Side Pockets: Yes. I would only need one, but I prefer a second for my fuel bottle as I get bit nervous about fuel spilling in my pack, and having two keeps things balanced.
Haul Loop: No thanks.
Closure: A cinch top is fine, but ideally with a waterproof pack fabric like cuben you'd also have a dry bag style roll top.