Sonic Mania's sequel was never developed due to Sega's concerns

Sonic Superstars is regarded as Mania's spiritual successor.

Sonic Mania, Awesome Games Wiki

Steam :: Rock, Paper, Shotgun :: Sonic Mania 2 was never in the works because Sega and its devs wanted to move away from pixel art and “make something fresh”

Sonic Superstars Had “a Slightly Weaker Start Than Anticipated” – Sega : r/pcgaming

Sonic The Hedgehog Series (Alternate Timeline), Sonic & Sega FanFiction Wiki

Sonic Origins Plus - A 'Dreamview' - What Could Have Been — cdahmedeh (Ahmed El-Hajjar)

Sega Reveals Why Sonic Superstars Was Made Over Sonic Mania 2

Sonic the Hedgehog technological and gaming history

Deluxe SEGA Genesis style Collector's Box 12 Classic Sonic Statue featuring SEGA Genesis Base Flip the power switch to trigger the SEEGAA startup

Sonic Mania: Collector's Edition - PlayStation 4

Sonic Mania Plus - PlayStation 4 : Sega of America Inc: Video Games

DF Retro: Sonic CD - under-appreciated but still brilliant today

Sonic Superstars Developers Explain Why They're Finally Abandoning Green Hill Zone - IGN

Sega explains why a Sonic Mania sequel didn't happen