Sketch made while explaining the six fundamental decisions that make a new Latin typeface hang together.
1) ratio of stroke thickness to x-height
2) width of key inside counters (in stroke multiples)
3) balance of inside and outside space (wrt style and optical size)
4a) shape of stroke joining verticals
4b) relationship of inside to outside of this stroke
5) height of visual line splitting x-height
6) grouping and range of widths.
1, 2, 3 are defined largely by genre and context of use. The rest is where design mostly happens.