I did a lil mod for myself: tunnels are "thinner" so that in fast songs the lvl doesn't look completly close with the massives iterations. I also added in the upper middle of the tunnel a swirl adding some kind of speed effect on the whole. ( ring2B)
I switched the hit texture with a star from mario (hit2) , and switched the particule effects with some old school enemies ; baron von blubba from bubble bobble , a metool from megaman , a flying goombas from paper mario , and a metroid from hum ... metroid ^^ (particule 2 and 3)
edit : a mod space invaders added (no screenshot)

the particule effect.

both the tunnels and hit effect.