Cool thread, I've been thinking about this too because machetes are so darn handy and don't look real hard to build. I recently picked up a cheap Harbor Frieght machete to mess around with till I get my ideas straight.
This is only what I've read, but I would look at 5160 for the steel. It's widely recommended for big blades that can take a beating, can be differentially tempered to help that, and takes a good edge. Although lots of perfectly good machetes have been made out of low-alloy steels (probably similar to 1075 or 1040?), and have lived through abuse just fine. I'd be curious to see how a more corrosion-resistant steel like D2 would hold up. In any case I think about 1/8" thickness would be strong, light and flexible enough to cut and slash real well.
The classic straight back, short-belly profile seems ok to me. Maybe 20% wider at the belly than at the hilt. For length I would think no less than 12", no more than 18". Even 18" seems kind of long to me unless you're going to be harvesting hay or something. Where I'm at, we have a lot more thick brush than we do tall grass so I don't need that much length. I'm going to start with 16" on mine and see how it handles, carries on my hip or pack, etc. I suspect I may end up cutting it down to 14 or 15".
I'm not a huge fan of those enclosed "D" handles, I guess I just haven't tried one that fit my hand quite right. It has to have a lanyard, preferably made of webbing material instead of paracord. So it doesn't cut into your hand. I like to use my lanyards wrapped under the back edge of my hand, across the back, and around my thumb. I learned that trick here somewhere and it's really secure but doesn't restrict your wrist if you adjust the length right.
An integral guard, and birds-head pommel are big plus too. Full-tang of course, epoxied and pinned. It has to be robust and not fall or slip out of your hand during hard work when you're all sweaty. I think mild finger-grooves and some checkering on any typical handle material would work.
There's a lot you could do with a machete sheath, there's so much material there to work with. I think at the minimum it should have a pocket for a diamond stone or small file. Maybe a small utility knife too, kind of how kukri's come with a small knife. You could go hog wild and make the sheath into a whole survival kit if you want. Someone around here just made a nice "plain" leather sheath for his Ontario, makes it look like a $200 knife
The first 3-4" of blade could be serrated; I'm not usually into that but I think it fits on a machete. Likewise, sawteeth like are found on folding saws, on the spine, would probably work pretty well. On thin stock you could even set a kerf to the teeth and make them cut like a real saw. Sounds like a lot of work but I think it could be done. All that ragged stuff on the blade means you'd need a hard plastic liner in the sheath.
You'd need a goog long forge to heat-treat the thing; maybe a fire built in a trough in the dirt would work? I've seen ones like that on other sites, pretty cool.
It occurs to me that I haven't said anything new at all
The basic design is pretty well figured out after all these years, I guess. Still waiting to see a nice satin-finished example with stag slabs on it though
