When doing the original spritework for a potential Super Mario Mania, I had to consider 3 things- where the mainline 2D Mario sprites were at previously, where they were at the time, and what they would become.
The art styles for the 2D Mario games were never consistent between titles. Barring the Lost Levels, each game style was radically different from the last due to Nintendo's nack for throwing everything out with each new installment. The only thing I really went off of was actually for colors and shading style- they're primarily taken from the Super Mario All-Stars versions of the original trilogy.
The time period Mario Mania intends to emulate is between SNES and N64, so around the Virtual Boy felt fitting. I took a lot from how Mario Clash was styled for how Mario and Luigi look, which also helped capture how 2D Mario art tends to look. The added detail for the eyes and boots go a long way!
Finally, I took into account what N64 pixel art (what few of it there was) looked like. Dr. Mario 64 was a huge help as well, and the general redesigns that came with the N64 era went into how the mushrooms, blocks, and enemies look.