Great idea. I don't know how widely acceptable is uniform 0.5u stagger. But I think it would be great if we can agree on it and a little more aggressive angle (60° instead of 53°): 6 full rows can fit in (5 * 13/15 + 1)u = 5⅓ u. (I use approximation sqrt(3)/2 ~ 13/15, with relative error 7.4E-4 and absolute error < 0.1mm for this size.) With proper key size, shape and orientation, the regular hexagonal grid can implement many good things, the Atreus included.

Example. A variation of the Atreus, with 60° (instead of 20°) opening angle and 0.5u (instead of 0.25u) column shift, in 5.2u × 10u form factor. (5.2 = 6*13/15 instead of 5⅓ because regular hexagons are used in place of circles.) The top surfaces of keys are 2u/3 (0.5") wide, ~10u/13 (~0.58") high and separated by a u/3 (0.25") gap. The bottom of keys are 8u/9 (2/3") wide.

A simpler, less sophisticated alternative would be chiclet key that can fit inside a 1u × 13u/15 'unit' rectangle, e.g. 18 × 15.5 mm key.