I've been thinking about tweaks and very small improvements to DF that would take almost no time at all to implement. Stuff we can do ourselves with raw modifications, or very very simple code changes.
Rule of thumb: If it would take more than one hour to implement, or you think it might take an hour maybe, it's not micro enough.
Examples: Lowering SHOOT_FORCE and SHOOT_MAXVEL to something more reasonable values(there is a lot of !!science!! that has been done on bolts and bolt impacts. Essentially, currently bolts travel so quickly they instantly punch through all plate. No plate provides any protection whatsoever. Lowering SHOOT_FORCE and SHOOT_MAXVEL to 25 and 250 produces less absurd behavior.)
Counter-example: Currently the weight of bolts is calculated as an integer. All bone/wood/adamantine bolts are rounded down to zero and have MAXVEL velocity every single time, because their weight is effectively zero yet they have finite momentum. Changing this to a floating-point number would involve changing I-don't-even-know-how-much code, and possibly invalidating a few optimizations. (strangely, for knockback mechanics, the weight of bolts is calculated in floating point. Adamantine is less effective than wood is less effective than bone)
So maybe that would be less than an hour for Toady One, but maybe more. It's a type change to an important variable, with potential side effects. So it's not a microsuggestion. Play it safe. Maybe a millisuggestion? Currently it's on mantis, anyway.
Historical microsuggestion: The implementation of accurate melting points for all the different kinds of rock.
Possible microsuggestion: Current leather production. There is
a hotfix for this one, which turns skin into a 'glob' and then tweaks the tan-a-hide reaction so that it consumes globs. It's hardly gamebreaking either way, but it weird to get a rhino hide and a turkey hide and get the same amount of leather out.
Anyway. Significance, esotericness, or controversiality be damned. Size and complexity of the change is all we care about in here. What is the smallest change you would like to see in DF?