Hard to say, really...I kind of like the idea of "TK"-size layouts, but I haven't seen any actual layout in that size category that looks right to me, and I don't know if it's even possible to do that size in a way that I'd consider good enough.
Failing the invention of a good layout of that size, I'd have to go for full size, preferably a layout with both a full-size backspace and big enter (put the \| on one or the other side of right shift, which I never use anyway). As for construction, I don't like aluminum. So: Steel plate, steel case (thick, not too big in terms of margins, but please, no sharp corners), thick PBT doubleshot keycaps. For features: give it protective covers over the switches (Azio-style), adjustable debounce time, high scan speed and polling rate, NKRO, MX switches with stabilizing pins (like for PCB mount, though this would have a plate as well).
Ideally, this would come with an accessory of a trackball wrist rest-- think iOne Libra-35, but better: it'd be the same width as the keyboard, with the padded portion of the rest done with the materials and construction standards of EK's 3-star leather wrist rest (or better), a higher quality trackball than in the Libra-35 with onboard sensitivity adjustment, and of course it would be made by a company who are better at making things than iOne.
