First, thank you Henry (Echinocactus123) for choosing my "wild card" photo in the August competition, it was pretty unexpected I must admit!
Back to this month's topic, I will go a bit selfish here, not in the sense of having any chances in winning it, but rather in looking for some inspiration to expand my cacti collection in a very specific direction:
Simple globes with pronounced tubercles or ribs
As a few quick examples, somewhat inspired by last week's Zoom talk (sourced from llifle.com encyclopedia)

Rebutia neumanniana var. kargliana:

Rebutia steinbachii:

and Gymnocalycium mihanovichii:

Fire up!