I might be able to help.

Sonic games used to be pretty straightforward: you ran fast, jumped high, spin attacked, and fight robots. But after making the jump to 3D, Sega has been giving him all kinds of crazy gimmicks, like turning into a wolfhog, sword, etc.

Rayman started out as high-quality 2D cell drawings, went to 3D models, then went back to 2D drawings.

Crash Bandicoot started off as blocky because of the technological limitations of the PS1, but gradually went towards a more "natural" look as graphics improved. There was supposed to be a reboot of the franchise that would have brought Crash back to his more cartoony roots as opposed to the direction Radical Entertainment took. Unfortunately for fans, the game was cancelled.

I don't know enough about Spyro, but if I had to guess, he's becoming bigger and more "dragon-looking" with each installment.

And finally, Banjo is becoming more blocky as time goes on.