The key to a 'one knife' is utilizing the one tool you have to make more tools that might be necessary to accomplish tasks. Obviously basic things like skinning, battoning, firecraft etc can be accomplished with a simple bushcraft knife, like a Mora, a machete, or anything in between. However felling trees, digging pits, carving detailed traps, etc may present a challange if you're limited to one tool.
It really comes down to your environment and your skill set-the materials you're presented with, the tasks at hand, etc. and your abilities to manipulate them. For some people it will be a Tramontina machete. Others will pick a scandi bushcraft knife, or a big thick E&E knife.
If not an axe, I'd take something in the 4.5-5 inch range, maybe .15 inches thick with a Scandi grind. L6 or 15N20 would be ideal, based on ease of sharpening and edge retention properties. In pine scratch, it wouldn't be hard to come up with a jo stick for producing dry dead low hanging branches for firewood. We're blessed with the world's best tooling obsidian, and all one needs is a core to produce a dozen prism blades, a good biface or even temporary-use flakes for taking care of detailed slicing tasks. We lack good hardwoods, so producing tools that require a strong haft or other tools that require consistant 'carvability' would be a challange. You might get lucky with cedar but otherwise you'd have to make do around here with the useless locust, cottonwood or softwoods. Where I grew up in Wisconsin, we had alot of flint-it was everywhere. Flint is more impact-resistant, longer lasting and harder to tool than obsidian, and you can manufacture chopping and splitting tools. With all the strong healthy hardwoods, you could fabricate a decent axe. In that case I'd take a thinner slicer.
