I've just acquired a compromise solution for my needs. Not a suitcase, slightly better than a duffel. Not a clam-shell. 2 inline-skate wheels. Concealed shoulder harness. Hard standing end (unlike most duffels).
Caribee Adventure 70L. And the only model I could see in a physical shop before buying!
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I haven't used it yet, but did a test pack. The two biggest downsides for me are that it tapers significantly from top to bottom, and the lack of a flattish surface at the top when standing means it would be hard to stack a small bag on top. Very different from my existing rectangular soft-side suitcase.
Of course, like pretty much every case ever, the stated volume is a fiction. I would put it at just under 60L when packed tight.
The upsides: Weight is reasonable for what it offers (3.4kg). All zips are lockable. Two front-face carry handles (but none on the long sides). The length adds some flexibility for transporting things like walking poles or interesting purchases. The wheels run smoothly and are slightly protected as not set flush with the bag's corners. Most people would not want to use the backpack harness as a regular thing for a bag this size, but it could certainly come in handy when negotiating lots of stairs.
For comparison, the other 2-wheel candidate bags I could find available in Australia without too much hassle and without abysmal reviews were all clamshell duffels. The biggest design issues with these are that the front half has no rigid foot/base or is just unsteady because that part isn't connected to the back half, and that a large clamshell is a pain to pack. None of the candidates had a front-face opening to get around the clamshell issue, unlike cheaper less-robust models. American Tourister Urban Pack: I felt the number of reported problems in reviews, although relatively small, seemed to point towards quality/design issues. Eastpak Transverz: Similar to the American Tourister, but with fewer reported problems; seems to have been replaced by a newer, slightly heavier, fancier model (CNNCT). Two more expensive candidates were High Sierra Composite 76 cm Wheeled Duffle V4 (getting too big, and single-pole handle), and the Osprey Transporter 60L (the material blemishes badly).
