O-Town member Erik-Michael Estrada joined us on "The TMZ Podcast" ... and said one of the most important factors in forming the perfect modern-day boy band is diversity.

He pointed out while bands in the '90s and '00s -- like his and Backstreet Boys -- were quite diverse, bands before them were frustratingly either R&B or pop ... there was no in-between.

Erik-Michael elaborated further on the importance of diversity within boy bands ... saying assembling members from different backgrounds ensures all fans will be able to relate to someone in the group.

He also stressed the importance of having what he calls boy band "slashers" -- people who can do a bunch of different things: dance, sing, play instruments, write songs ... you name it.

Check out the clip to see how he addresses one other fact of boy bands -- namely that one member eventually becomes the leader or main singer. While that can, obviously, cause tension, or a rift, he says there's a way to avoid that before things go south.

As we reported, Simon is on the prowl for his next pop success story, 15 years after assembling 1D on the "X Factor."

The former "American Idol" judge explained the reason he's on the hunt for his new megastars is 'cause there hasn't been an impactful boy band since One Direction.