What is 5/5/2 variant of the Machi Koro?
This variant seems to have been introduced with the Harbor Expansion and I'm not sure it would work well with just the base game. It's a way to avoid having only low (resp. high) numbered buildings when using the draw cards rule (instead of having all the buildings available at once).

The idea is apparently to split the deck into 3 decks :


one with the buildings 1-6 (except purple ones)
one with the buildings 7-14 (except purple ones)
one with all the purple buildings.


Now, when drawing the cards to prepare available buildings, you draw from the first deck until you have 5 different buildings, from the second one until you have 5 different buildings and 2 different purple buildings.

During play whenever someone depletes a pile, draw from the corresponding deck until you reach the wanted number of buildings in this category (5, 5 or 2).

I didn't know about this variant and this made me curious... I found about it in this forum thread on BGG.

I might as well give it a go myself ;-)
Thanks for a good question!