sonic games have a really hard to pinpoint asthetic. in the first two id say make everything that isnt moving polygonal and everything that is moving pretty round(except loops)
as far as for all the originals the stuff ive noticed that repeat are
1 ancient places with modern colors, or modern places with retro tiles. the best examples are the bright purple stone work in the background of hydrocity zone or the super weird deco building tiles on the stace station in sonic adventure 2
2 industrial levels, if there water itll kill you, lots of steel with yellow/black stripes really hard color contrast like the purple green and orange of oil ocean. blue orange and yellow of chemical plant. both have a pale green in there too(checks scrap brain for pattern... yeah pale green seems to be there alot..(wouldnt use that to tie you down though)
3 floating lights and checkerboarding for decoration... ..
....... man it a pain pinpointing a pattern with those games. look up 90s graphic deign for alot of pretty simialr looks to sonic games or this style is prettymuch the sonic level title screens in furniture form http://gizmodo.com/why-a-once-hated-1980s-design-movement-is-making-a-come-1602111413?utm_campaign=socialflow_gizmodo_facebook&utm_source=gizmodo_facebook&utm_medium=socialflow