Most of the kit are featuring the secondary colour from the keyset, sometime with additionnal keys for colour accent or text label.
The bare minimun is 12 * 1.5u (4 vertical, 8 horisontal) 4 vertical 2u for thumbs, 20 * 1u.
I think you could pick between two common approach :
Mod keys with labels
Not the easiest way as a lot of ergo users are heavily customizing their keymaps to their needs.
The defaut layout in QMK is the default
Ergodox EZ one.
Some keycaps creators are trying to provide some labeled keys with usually several options.
Examples:
dsa miami, dsa light cycle,
dsa granite,
Nautilus (lot of options there)
Some provide labeled keys with thematic text unrelated to key function:
Examples:
mt3 godspeed,
kam wraith, etc.
Blank keycaps
Obviously the main choice here.
Examples:
Mt3 Susuwatri (simple kit with a few colour accent option),
dsa royal navy (minimalist kit),
SA Vilebloom,
gmk terminalAs I'm not a designer I couldn't help you further, I guess you'll have to pick a solution.
My advice if you stick to labeled keys, keep colour pattern consistent and offer a few key variations.