Season 4's Beast Spirits inspired by season 2?
 

I was just looking at the Raidramon pic on my desktop, and it made me think of something. In season 2, the Digi-Eggs allowed for each Digimon to Digivolve into two completely different forms. Veemon could become Flamedramon, which was bipedal (two-legged), or Raidramon, a quadroped (four legs), which Davis would normally ride around on. Same for Halsemon/Shurimon, and Digmon/Submarimon. Heck, even the Digi-Eggs gave Patamon a horse form, and Gatomon a sphinx form, as opposed to their normal angel Digivolutions.

It looks as though season 4 making up the human and beast spirits angle was just a throwback to what season 2 had already established.

RE: Season 4's Beast Spirits inspired by season 2?
 

Probably, I think a lot of Frontier was them going back to adventure/02 taking little sniglets here and there.