My favourite one's this sketch page.

There's something nice when artists do those. It's neat to see different ideas done so quick, and they also look like they could be stickers in the future, which is fun.

I'm biased, but muscles would be cool to see. They're fun to draw and adapt to characters.

I'd recommend this one. The VA is not great and I don't remember if the story is any good, but I used to like the gameplay a lot. Think of it as Bomberman but you can plant bombs at a distance and snap them to enemies.

Not exactly a challenge-tober, but you should look at MtKanjon's drawing calendars. Although he will be taking a break from making those on December, there are plenty to choose from here: mt.kanjon.art/calendars/. Last month was about anatomy and this month's focused on landscapes