Square legos would work reasonably well, with some caveats:
Firstly, I checked
https://en.wikipedia.org/wiki/Lego for the text "legos", and it could not be found.
(In my opinion, "lego" is like "sand". When not fully described as "LEGO
® bricks"
1 , with or without specific capitalisation and/or trademark acknowledgement, you must usually treat it as an uncountable mass-noun. And if you don't have enough Lego for it to be uncountable, you don't have enough!)
1 Or "...pieces/blocks/whatever".
You will need LOTS of space.
you will need LOTS of blocks.
I suggest 2x2 pips per DF 'block'.
Vertically, define eight 'thirds' per full-Z (bottom-most being natural/constructed floor, if any, with (above a 'spacer'/not-floor piece, where necessary) up to seven further thirds of liquids (red or blue translucent pieces for magma/water), vapours (clear for mist, purple for miasma, etc) or a full seven thirds-worth (could also be a full-thids-full stack, a third-third-full-third-third one, etc), of mixed colours to represent natural/constructed material;
Flowing water can be represented using 2x2 slope-pieces of various kinds, or even dot those 1x1 'third-with-sloped-flat-tops' around, to taste;
Ramps can involve various combinations of double-height 2x2 slopes (of less than 2x2 top-pips) or else standard-height 2x2-based slopes (ditto) with the sloped-flat-topped-thirds covering the pip-tops, whilst stairs use stacked 2x1-based/1x1-topped slopes in an overhanging spiral, with or without spacers.
Use 2x2 circular blocks for (constructed/smoothed) wall-ends, especially two wall-ends that are connected only diagonally, through which creature-passage/liquid-flow would be expected to be observable.
All kinds of obvious furniture can be constructed: e.g. single-pip-round set in the mid-point of the four floor-pips with a 2x2-round-third centred astride atop would be a table; 2x2 third-height turntable used as the base for rotatable doors; hinge-pieces (especially the 1x2 standard-height composite) used to create floodgates.
And, above all, at this scale, various available disembodied minifig heads (especially 'themed' ones, with beards) can represent dorfs and other sentients. You can top'n'tail them with 'flavour' pieces (coloured to indicate race/job/martial-speciality), like various spaceman-helmets for militarisation. I've just checked, to make sure, and standard minifig heads (like 1x1 'rounds') nicely interstitially click into the central gap of a 2x2 brick-top. (Although a disembodied 'Yoda-head' theme headpiece does
not, so I unfortunately can't suggest a bucket full of yoda-heads on stand-by for the next goblin siege diorama, unless you are already prepared to put the head atop a handy 1x1-round (third/full) that
would fit betwixt the two.)
(I
have thought a lot about this. Mostly I've thought "They've done Minecraft in Lego, with quite a lot of artistic licence with regards to geology/architecture, so why not DF?"...)