Obviously the natural extension is to each 2-dimensional subspace of n > 2 dimensions, i.e. the y-z and x-z planes... etc.

(Ideally my interviewer starts laughing with me here)

Out of interest, what would you consider to be a good reply?

The 'obvious' answer is to have a second glyph (and third, etc.), but that's rather a boring answer.

Perhaps making a cross-shape so you have four-arms to work with (and can thus get to 10.000.000).

Or making a capital H shape would let you have the next four digits, which could then be extended out indefinitely; |-|-| etc.

You could use multiple dimensions. Or colours. Or fractals. Or any one of a hundred things.

But the real answer to any "how would you...?" questions is to reply with "Who are the users and what are their needs?"

Well, another obvious extension would be to prolong the vertical line, since it's already ordered by rows (two of them).

Or, indeed, you could make an array out of them, making a super-glyph (kind of like a block matrix). Four blocks of four digits would get you to 10^4^4 = 10^16 so 100,00,00,00,00,00,00,00 (or 10000,0000,0000,0000, or 10,000,000,000,000,000 depending on your grouping), which is 10 Quadrillions in short- scale. Obviously grouping digits by blocks of 2 or 4 makes mor sense, since that's a row or a glyph.

Interesting discussion though, which is probably the goal here, rather than a good or bad reply: it allows you to see how the candidate thinks or approaches a problem.

It’s intentionally open-ended to engage in discussions about functional and quality attributes. Does the candidate ask questions or just jump into an answer?

I love it because it’s very open-ended and demonstrates problem solving. It’s a great conversation piece.