An alternative to the conventional leg based on a leotard style pattern, this involves separate body and leg pieces. It's possible to combine a front in this style with a back in conventional style, or vice-versa.
- Plot the four-panel bodysuit pattern, and make two copies of the crotch area, from navel (or wherever the crotch curve starts on the centre front line) to mid thigh. Be sure to copy the centre front line (dotted red vertical on top diagram) to both.
- On the first copy (the leg copy) draw a 'dropped crotch line' horizontally, 1.5 inches below the crotch point and twice the length from the inside leg seam to the centre front line. From the end of that line, draw another line at 45 degrees until it meets the side body seam. Call this point A. (1.5" should be about half the distance between your legs at the top.)
- Make a smooth curve (the groin curve) from the intersection of the dropped crotch and the centre front line to the 45 degree line.
- With both copies of the crotch area atop each other, copy point A from the leg copy to the body copy. Also draw a line (the shifted centre front) parallel to the centre front line, but shifted by the amount the dropped crotch line was dropped. (See lower diagram)
- Now flip the leg copy over so the groin curve's A is still at A, but its other end points down the leg, and rotate it around A until the end pointing down the leg meets the shifted centre front line, at C. Draw a horizontal across from C to meet the centre front line (green), then draw up the centre front line to cut off the crotch curve (green).
- You should end up with the body piece looking like half a leotard (ACgreen), and the leg piece like a leg with a matching (inverse) curve (AB). There shouldn't be much of a loop at A (see diagram), if indeed any at all.
- Add seam allowance to these two curves. (At least enough to make up for the stitch width.)
With suitable account taken for sleeves and collar:
- Cut one front body panel with what was the centre front line on a fold.
- Cut two leg pieces with wrong sides together.
- With wrong sides together, sew the leg curve AB to the matching leotardy curve AC for both legs.
- Construct your back body half, and sew front to back.
Since the bottom of the leotardy body panel works in much the same way as underpants, it's possible to put a fly into it. You could duplicate a Y-front style arrangement, or make a vertical lap, but you may need to put some sort of closure - pops or buttons - to help keep it closed if it's under enough tension to pull open.
I'll try to write this up sometime.